When chat that connected to channel reaches 1 million posts they going to "archive" and become unavailable so all comments on channel posts is gone too.
At the same moment channel posts that "pinned" in connected chat is still available ("Leave comment" button is still here on all old posts).
Example of chat that have this limit: https://t.me/ru2chnews/3500035 - first post in October https://t.me/ru2ch_news/20 - but connected channel have posts in September
And all comments is gone because of that posts limit in chats.
Very likely. Topics are just another way to manage the messages stored in the associated databases. Also, enabling topics does NOT increase the quota. THE MESSAGES ARE STILL IN ONE DATABASE (checked that through the message ID's).
D
Deleted Account
if telegram does not want to make it available to everyone to save money, it could add it to the functions for telegram premium users
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