The developers said that they received an email from Telegram, in which they were asked to add an official advertisement from January 1, 2022. Prior to that date, client creators must ensure that the app supports and correctly displays advertisements. If the developers do not do this, Telegram will disable API access for such clients.
Some app makers have already agreed with this. Plus Messenger on Android already has ads. The developers of the iMe Messenger client said they saw “nothing problematic” about supporting Telegram through displaying ads.