Repository: yt-dlp
Issue: [YouTube] Wanna Download Thumbnails Used in A/B Testing
Labels: site-enhancement,triage

### DO NOT REMOVE OR SKIP THE ISSUE TEMPLATE

- [X] I understand that I will be **blocked** if I *intentionally* remove or skip any mandatory\* field

### Checklist

- [X] I'm requesting a site-specific feature
- [X] I've verified that I have **updated yt-dlp to nightly or master** ([update instructions](https://github.com/yt-dlp/yt-dlp#update-channels))
- [X] I've checked that all provided URLs are playable in a browser with the same IP and same login details
- [X] I've searched [known issues](https://github.com/yt-dlp/yt-dlp/issues/3766) and the [bugtracker](https://github.com/yt-dlp/yt-dlp/issues?q=) for similar issues **including closed ones**. DO NOT post duplicates
- [X] I've read the [guidelines for opening an issue](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#opening-an-issue)
- [X] I've read about [sharing account credentials](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#are-you-willing-to-share-account-details-if-needed) and I'm willing to share it if required

### Region

Japan

### Example URLs

https://youtu.be/u8s6Jkl3Mog (Nobamangames presents as Tomasuko)
1. https://i9.ytimg.com/vi/u8s6Jkl3Mog/hqdefault_custom_1.jpg?sqp=CLCQibgG-oaymwEcCNACELwBSFXyq4qpAw4IARUAAIhCGAFwAcABBg==&rs=AOn4CLBbjfNZkih_IxDleZqbneNnXwnavA (Art by にとの)
2. https://i9.ytimg.com/vi/u8s6Jkl3Mog/hqdefault_custom_2.jpg?sqp=CNiLibgG-oaymwEcCNACELwBSFXyq4qpAw4IARUAAIhCGAFwAcABBg==&rs=AOn4CLAC9kpCNN_YJMCdrjvDpvuF369bUA (Art by えびな)
3. https://i9.ytimg.com/vi/u8s6Jkl3Mog/hqdefault_custom_3.jpg?sqp=CNiLibgG-oaymwEcCNACELwBSFXyq4qpAw4IARUAAIhCGAFwAcABBg==&rs=AOn4CLCre_b2ZggdExYQoAVCHVTuMVaWTA (Art by たんすのもやし)

### Provide a description that is worded well enough to be understood

YouTube introduced the [Test & Compare](https://support.google.com/youtube/answer/13861714) feature, which uses a maximum of three thumbnails.
Now `yt-dlp` doesn't recognize them, so I want it to.

For example, I found them all in his video list.
Their resolution is `hqdefault`, like normal thumbnails,
but they're on i**9**.ytimg.com, **and** u can't get images without any URL parameters.

And the official help page I attached above says
> If the resolution of any thumbnail is lower than 720p (1280 x 720),
all experiment thumbnails will be downscaled to 480p (854 x 480).

So I'm wondering if `sddefault_custom_*` or `maxresdefault_custom_*` exists.

Thx for ur consideration.

### Provide verbose output that clearly demonstrates the problem

- [X] Run **your** yt-dlp command with **-vU** flag added (`yt-dlp -vU <your command line>`)
- [ ] If using API, add `'verbose': True` to `YoutubeDL` params instead
- [X] Copy the WHOLE output (starting with `[debug] Command-line config`) and insert it below

### Complete Verbose Output

```shell
[debug] Command-line config: ['-vU', '--list-thumbnails', 'https://youtu.be/u8s6Jkl3Mog']
[debug] User config "C:\Users\atsus\AppData\Roaming\yt-dlp\config.txt": ['-N', '20', '--console-title', '--embed-thumbnail']
[debug] Encodings: locale cp932, fs utf-8, pref cp932, out utf-8, error utf-8, screen utf-8
[debug] yt-dlp version master@2024.10.01.001408 from yt-dlp/yt-dlp-master-builds [e59c82a74] (win_exe)
[debug] Python 3.8.10 (CPython AMD64 64bit) - Windows-10-10.0.22631-SP0 (OpenSSL 1.1.1k  25 Mar 2021)
[debug] exe versions: ffmpeg 7.0.2-full_build-www.gyan.dev (setts), ffprobe 7.0.2-full_build-www.gyan.dev
[debug] Optional libraries: Cryptodome-3.20.0, brotli-1.1.0, certifi-2024.08.30, curl_cffi-0.5.10, mutagen-1.47.0, requests-2.32.3, sqlite3-3.35.5, urllib3-2.2.3, websockets-13.1
[debug] Proxy map: {}
[debug] Request Handlers: urllib, requests, websockets, curl_cffi
[debug] Loaded 1838 extractors
[debug] Fetching release info: https://api.github.com/repos/yt-dlp/yt-dlp-master-builds/releases/latest
Latest version: master@2024.10.01.001408 from yt-dlp/yt-dlp-master-builds
yt-dlp is up to date (master@2024.10.01.001408 from yt-dlp/yt-dlp-master-builds)
[youtube] Extracting URL: https://youtu.be/u8s6Jkl3Mog
[youtube] u8s6Jkl3Mog: Downloading webpage
[youtube] u8s6Jkl3Mog: Downloading ios player API JSON
[youtube] u8s6Jkl3Mog: Downloading web creator player API JSON
[debug] Loading youtube-nsig.96d06116 from cache
[debug] [youtube] Decrypted nsig kkZZG4vz950ORK5Qn9 => rgKFTTb8FVfHew
[youtube] u8s6Jkl3Mog: Downloading m3u8 information
[debug] Sort order given by extractor: quality, res, fps, hdr:12, source, vcodec:vp9.2, channels, acodec, lang, proto
[debug] Formats sorted by: hasvid, ie_pref, quality, res, fps, hdr:12(7), source, vcodec:vp9.2(10), channels, acodec, lang, proto, size, br, asr, vext, aext, hasaud, id
[info] Available thumbnails for u8s6Jkl3Mog:
ID Width   Height  URL
0  unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/3.jpg
1  unknown unknown https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/3.webp
2  unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/2.jpg
3  unknown unknown https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/2.webp
4  unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/1.jpg
5  unknown unknown https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/1.webp
6  unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/mq3.jpg
7  unknown unknown https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/mq3.webp
8  unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/mq2.jpg
9  unknown unknown https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/mq2.webp
10 unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/mq1.jpg
11 unknown unknown https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/mq1.webp
12 unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/hq3.jpg
13 unknown unknown https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/hq3.webp
14 unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/hq2.jpg
15 unknown unknown https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/hq2.webp
16 unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/hq1.jpg
17 unknown unknown https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/hq1.webp
18 unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/sd3.jpg
19 unknown unknown https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/sd3.webp
20 unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/sd2.jpg
21 unknown unknown https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/sd2.webp
22 unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/sd1.jpg
23 unknown unknown https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/sd1.webp
24 unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/default.jpg
25 120     90      https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/default.webp
26 unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/mqdefault.jpg
27 320     180     https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/mqdefault.webp
28 unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/0.jpg
29 unknown unknown https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/0.webp
30 168     94      https://i.ytimg.com/vi/u8s6Jkl3Mog/hqdefault.jpg?sqp=-oaymwEbCKgBEF5IVfKriqkDDggBFQAAiEIYAXABwAEG&rs=AOn4CLBwWqiJVaIr7DBkoQNarLWc4XQOKA
31 196     110     https://i.ytimg.com/vi/u8s6Jkl3Mog/hqdefault.jpg?sqp=-oaymwEbCMQBEG5IVfKriqkDDggBFQAAiEIYAXABwAEG&rs=AOn4CLAh6RsUGxxNs3j4VrD7gC49q4prlg
32 246     138     https://i.ytimg.com/vi/u8s6Jkl3Mog/hqdefault.jpg?sqp=-oaymwEcCPYBEIoBSFXyq4qpAw4IARUAAIhCGAFwAcABBg==&rs=AOn4CLBfwLTx2rmbHAwwELzyNbT9fCA-7g
33 336     188     https://i.ytimg.com/vi/u8s6Jkl3Mog/hqdefault.jpg?sqp=-oaymwEcCNACELwBSFXyq4qpAw4IARUAAIhCGAFwAcABBg==&rs=AOn4CLBdpUreLBiRDGASKQkxCYAv_CFnvA
34 480     360     https://i.ytimg.com/vi/u8s6Jkl3Mog/hqdefault.jpg
35 480     360     https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/hqdefault.webp
36 640     480     https://i.ytimg.com/vi/u8s6Jkl3Mog/sddefault.jpg
37 640     480     https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/sddefault.webp
38 unknown unknown https://i.ytimg.com/vi/u8s6Jkl3Mog/hq720.jpg
39 unknown unknown https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/hq720.webp
40 1280    720     https://i.ytimg.com/vi/u8s6Jkl3Mog/maxresdefault.jpg
41 1920    1080    https://i.ytimg.com/vi_webp/u8s6Jkl3Mog/maxresdefault.webp
```