[OnePlus 10 Pro] yo-less:
Hi @dladz, I ran into a similar question three years ago when I bought a Xiaomi Mi 8 Pro. I was so used to using custom ROMs that I didn't even check before I bought it whether any custom ROMs would be available.

I was honestly shocked to see that that wasn't the case - there was absolutely NOTHING available - and that I had ended up with a device that couldn't even be unlocked right away, you had to wait and open a Mi account to be allowed to unlock it. I couldn't believe it at the time, I had come from a OnePlus X and had helped others root and install custom ROMs on other OnePlus phones without any problems, so I felt like I had just thrown quite some money out the window to run around with a mobile Chinese Spyware gadget I didn't want to use.

That was the first time I ever came across the term GSI ROM and it took me several weeks to figure out how to flash a ROM that I could also use afterwards. I knew I wouldn't want to have a stock ROM by a company that has very loose data privacy standards and even clearly states they collect your data (Lesson learned, I thought, I just had to buy a "proper" phone again and no more phones from budget manufacturers).

Several weeks later, after I had finally managed to get a custom GSI ROM working on my device I posted a lengthy how-to here on XDA.

Again, it took me weeks to get everything to work that I could not go without. And even then, I had to resign to the fact that many features of the phone would not be working (The fingerprint sensor, for example. Also, the Xiaomi Mi stock rom apparently had excellent GPS support (dual GPS support) but GPS positioning is just alright on the GSI ROM I'm using, also there were many cool Xiaomi apps like a compass app that wouldn't work outside MIUI). But I was more than happy I had gotten different GSI ROMs to work in the end and settled on one that I could live with, knowing my phone wouldn't be sending data to some Chinese servers constantly.

Now that the Mi 8 Pro has become a bit slower over time with all the latest apps, I took the plunge and spent a ton of money on the OnePlus 10 Pro 256 GB version only to be disappointed again, but this time by a company that used to be the epitome of openness when it came to custom development.

I got my OnePlus 10 Pro on release in my region in April, together with the 50W wireless charger and both have been sitting here since April, I'm not touching that phone until I can remove all the Spyware it comes installed with. At this stage, I'm not yet willing to go through the weeklong trial-and-error again that it took to get the Mi 8 Pro running a custom GSI ROM (also considering that some of its features never worked...), but if OnePlus doesn't get its **** together soon with the MSM tool and some openness towards developers, I will seriously just consider selling it on eBay again (It is brandnew after all) and will never buy another phone from OnePlus again. This has been an extreme disappointment and I'm still struggling to understand why they wouldn't support custom development any more. I had checked the OnePlus 9 Pro threads on XDA just to see if there's still custom development going on for OnePlus and it seemed everything was still OK, but now... .

On a more positive note: The Mi 8 Pro did eventually get some custom ROM love and there are now several ROMS to choose from, so to answer your original question - If you ask me, don't bother with GSI ROMs until we can be sure there won't be any custom ROMs for our phone. I'm still hoping that will be the case.