I need to be invited to more Teams ;) But it does raise the question - why is it not available in 1:1 chat? It is a bit like disabling email Subject if not being sent to a DL. I guess it is for SfB backwards compatibility reasons?
Again I refer to my email analogy. If I send just you an email I will fill in the Subject line in the same way as if sent to a DL with a 100 users. If I look at my Chat pane it is full of chats to 2 individuals with their name and 1st line of the chat displayed. This means I often have to open the chat to find the exact one I want to reply/refer to. Even hovering over it doesn't give a flyout box.
Regarding your comment "Chats on the other hand are threadless". They are persistent (unlike SfB IM which timeout to Saved Conversations) which means I can go back and reply to a chat at any time directly from the sidebar. This is why a Subject line would be a useful option instead of having to work out which of the 20 chats to John Doe I want to reply to.
Please remember - there are still people using a PC with a decent size screen that can accommodate an extra line or two. Thankfully we are not all forced to use a mobile device yet ;)