From what I’ve seen, there are some blobs. At least Telegram-FOSS says:

Several proprietary parts were removed from the original Telegram client, including Google Play Services for the location services, HockeySDK for self-updates and push notifications through Google Cloud Messaging. Location sharing functionality is restored using OpenStreetMap.

Same page is where I learned you cannot register from third-party clients btw. Not nearly as big of a blow as removal of desktop registration, but still gross that you’d have to touch a partially-proprietary official app first.

Desktop client does not even have e2e, lol. (I don’t know if there are third-party options that do).

Yeah, if I am not mistaken, this has happened in Iran - the registration confirmation messages just wouldn’t arrive.

I am mostly concerned about potentially needing specific Big Tech implementations for them in some way… I don’t mind using, say, KeepassXC for it, because it is independent from any account or hardware, as well as easily backupable. But NOT anything tied to a Google or MS account.

Maybe I am misunderstanding something, but Paypal says it restricts what passkeys can be used, so it is apparently possible:

Passkeys are currently available for eligible personal accounts. An eligible Apple or Android device is required to create a passkey.