They are no longer a messaging/communication platform - now they are an advertisement platform. Just like Viber, for example.
New versions of Telegram for at least a month or so are testing forced ads. And they are even planning to add a fucking SUBSCRIPTION to disable ads.
And ads are not something that is pushed by channel/group owners or something - they are pushed by Telegram, and they appear independently of channel/group content. At the moment they are only testing with text-only ads, but we all know where it goes.
So, of course, they don't want you to use some outdated or modified client that allows you to bypass their ad shilling.
Telegram is dead and finished. Good night sweet prince.