If you want to get "super technical" about it, this feature was introduced years ago in the original app but taken away around v3.0.
EDIT: Since I do not have Grindr Xtra, if someone would like to actually send me screenshots of what this feature looks like, I may be able to re-work it so that the module provides a similar feature that is not a direct replica of the paid feature.
Don't have Xtra, but something to bring back that function making it different enough might be to have a little circle with counter in it: currently app gives you a yellow circle with the number of new messages. You could play with that idea: just a yellow dot if there are viewed messages, or include the number (?).
Along the same lines, I have a suggestion: In my area you often get a few users who don't upload a photo and you just see the generic black profile pic with no user name. Would there be any harm / Would it be possible to give those profiles a random name (user1123) so as to know which user is which?
I'm still used to swiping left/right to switch between profiles and inbox. It's annoying you actually have to tap on those now. Dunno if you can revert that behavior.
Say you're chatting with someone and you want to go back to "nearby" view. you have to go back and then tap on "nearby". It might be interesting to have that navigation bar with nearby/inbox/favs persistant throughout the app, with it autohiding and showing back up when you scroll down. This way there's an easy way to always go back to home, and we also always see if there's a new message notification.
Another suggestion I had way back when was to further customize this navbar: for example, I'd love to order it to have first favs, then nearby, then inbox, and being able to swipe between the views. At this point it might also be nice to have the option of having the navbar either on the (new) bottom or (old) top.
Sorry for the long post! Thanks for your efforts, and it's also fun to think about UI design like this!