Fastmail by default uses @fastmail.com addresses for masked email, same as their cheapest plan, so when I used it there was never anything blocked except the occasional site that only accepts gmail addresses (yep, they exist). Personally I prefer it that way for personal use, the custom domain sacrifices anonymity, but it might be nice for business users.
Fastmail is still like 0.1% of users. If you're a "growth & engagement" company you're probably better off banning that domain wholesale and as a bonus will get rid of tech-savvy ad/tracker blocker users without making any impact to your target market.
The advantage of iCloud is that it's a domain laymen use - those same laymen the "growth & engagement" scum wants to track and spam. They can't just ban it wholesale without alienating a large chunk of their target market (and a pretty lucrative one at that, since Apple hardware is expensive).
A while ago I ran into the first site that told me I couldn't use my fastmail masked email: remove.bg.
I don't know if they block every fastmail.com address, or if they somehow check if it's a masked email.
