The standard I recall being established back in the nineties as to whether strong encryption was even legal in the US was “substantial non-infringing use” or similar. It’s been awhile.
The problem with key-escrow or anything similar is that any proscribed circumvention is also available to the “bad guys”.
I think Telegram’s stance would be that they can’t moderate because of strong end-to-end encryption. Back in the day the parallel would have been made to the phone system or mail.
Of course this is all happening in France, so I have no idea what the combination of French and EU laws will have on this, but I would still broadly expect that if a parallel can be made to mail or phone, Telegram would be in the clear. The phone company and mail service have no expectation of content moderation.