I was positively stunned to learn such a thing, so while I don't have iOS I was able to find some references to that behavior in discussions.apple.com which led me to the fact that Safari on macOS can do it, too

But Apple gonna apple about using the plist hammer for every file format nail in their world, so no: definitely not the same thing we are discussing here

$ xxd -l 512 'You can also generate warc natively in iOS! On a website, tap the share sheet ic... | Hacker News.webarchive'
00000000: 6270 6c69 7374 3030 d201 0203 205f 100f bplist00.... _..
00000010: 5765 6253 7562 7265 736f 7572 6365 735f WebSubresources_
00000020: 100f 5765 624d 6169 6e52 6573 6f75 7263 ..WebMainResourc
00000030: 65a5 040d 1216 1bd4 0506 0708 090a 0b0c e...............
00000040: 5f10 0f57 6562 5265 736f 7572 6365 4461 _..WebResourceDa
00000050: 7461 5f10 1357 6562 5265 736f 7572 6365 ta_..WebResource
00000060: 4d49 4d45 5479 7065 5f10 1357 6562 5265 MIMEType_..WebRe
00000070: 736f 7572 6365 5265 7370 6f6e 7365 5e57 sourceResponse^W
00000080: 6562 5265 736f 7572 6365 5552 4c4f 111c ebResourceURLO..
00000090: dd62 6f64 7920 207b 2066 6f6e 742d 6661 .body { font-fa
000000a0: 6d69 6c79 3a56 6572 6461 6e61 2c20 4765 mily:Verdana, Ge
000000b0: 6e65 7661 2c20 7361 6e73 2d73 6572 6966 neva, sans-serif

$ plutil -p - < 'You can also generate warc natively in iOS! On a website, tap the share sheet ic... | Hacker News.webarchive'
{
"WebMainResource" => {
"WebResourceData" => {length = 5725, bytes = 0x3c68746d 6c206c61 6e673d22 656e2220 ... 3e3c2f68 746d6c3e }
"WebResourceFrameName" => ""
"WebResourceMIMEType" => "text/html"
"WebResourceTextEncodingName" => "UTF-8"
"WebResourceURL" => "https://news.ycombinator.com/item?id=39185542"
}
"WebSubresources" => [
0 => {
"WebResourceData" => {length = 7389, bytes = 0x626f6479 20207b20 666f6e74 2d66616d ... 64656e20 7d0a7d0a }
"WebResourceMIMEType" => "text/css"
"WebResourceResponse" => {length = 1880, bytes = 0x62706c69 73743030 d4010203 04050607 ... 00000000 0000065c }
"WebResourceURL" => "https://news.ycombinator.com/news.css?15bzD7gVH5AOktnbu3y0"
}
1 => {
"WebResourceData" => {length = 131, bytes = 0x3c737667 20686569 6768743d 22333222 ... 2f3e3c2f 7376673e }
"WebResourceMIMEType" => "image/svg+xml"
"WebResourceResponse" => {length = 1842, bytes = 0x62706c69 73743030 d4010203 04050607 ... 00000000 00000636 }
"WebResourceURL" => "https://news.ycombinator.com/triangle.svg"
}
2 => {
"WebResourceData" => {length = 315, bytes = 0x3c737667 20686569 6768743d 22313822 ... 2f3e3c2f 7376673e }
"WebResourceMIMEType" => "image/svg+xml"
"WebResourceResponse" => {length = 1842, bytes = 0x62706c69 73743030 d4010203 04050607 ... 00000000 00000636 }
"WebResourceURL" => "https://news.ycombinator.com/y18.svg"
}
3 => {
"WebResourceData" => {length = 43, bytes = 0x47494638 39610100 010080ff 00c0c0c0 ... 00000202 4401003b }
"WebResourceMIMEType" => "image/gif"
"WebResourceResponse" => {length = 1826, bytes = 0x62706c69 73743030 d4010203 04050607 ... 00000000 00000626 }
"WebResourceURL" => "https://news.ycombinator.com/s.gif"
}
4 => {
"WebResourceData" => {length = 5224, bytes = 0x66756e63 74696f6e 20242028 69642920 ... 636c6963 6b293b0a }
"WebResourceMIMEType" => "application/javascript"
"WebResourceResponse" => {length = 1945, bytes = 0x62706c69 73743030 d4010203 04050607 ... 00000000 0000069b }
"WebResourceURL" => "https://news.ycombinator.com/hn.js?15bzD7gVH5AOktnbu3y0"
}
]
}