I use "Saved Messages" in Telegram for this purpose. Very handy because you can forward messages from other users or channels.
Tagging is different from forwarding in obvious ways (one can delete chats, modify messages, etc), but sometimes it is even more convenient than clumping up your Saved Messages. Tags may be used there as well, obviously.
There is another method, somewhat abusive to telegram, but it doesn't seem to care. For a specific topic, you can easily create a bot with a unique name and just chat with them. It is okay if bot doesn't actually have a backend and doesn't read your messages. Also bot API is so easy to use, so that if you're a developer and have a personal vps or "underbed server", you can connect yourself to a database of any flavor you could imagine. Telegram really shines here, and an entire $subj functionality may be implemented as such bot (which maybe is a great opportunity to promote and integrate your app further, $author).