My thoughts on this here
https://techcommunity.microsoft.com/t5/Microsoft-Teams/Mark-as-unread-in-Teams/m-p/229159

Read the link above - or I'll quickly explain here again

I use "Mark unread" quite a lot in the Teams Activity feed.

But there's also a "mark unread" feature on each post in the Conversations tab - and in my mind it does nothing.

Well it does two things:
1. it bolds the channel
2. and it puts a link above the post saying -----Last Read----

But when you click out of the channel and then click the bold channel again, it doesn't take you to that post you marked unread - it just takes you to the most recent post

And what if I marked multiple posts as unread within a channel - how would it show me the 4 posts I have marked unread in this channel
because from the use experience this is what I expect it would do, but it doesn't

Please tell me if it's me failing in this user experience...as I'm totally not seeing something here

When I first saw mark unread on each post in the conversation tab I thought it was a bit like Yammer where you can manually push a post into your Inbox (follow in inbox). But it doesn't do this ie. when I mark a post unread in the conversation tab it does not put it in my activity feed and bold it