since yesterday, Telegram removes channels with no content posted since a year
the principle is to freed some usernames
no one have been noticed about that, not even the channels owners
this morning, making a lot of global searches for the channel username would make you able to take it back (see under screenshot)
however, it’s no longer possible since few hours (excepting for premium users 💀)
no one knows if it’s a bug or a new feature

also the public usernames system may be changed soon

SO ! post things on your old channels, or then :
• they will be gone
• the content inside will also be gone
• no one would be able to claim the username before a while