Don't worry there. TS2 wasn't dead the day TS3 was released either. People will likely have plenty of time to switch over before they decide to discontinue support for TS3, especially since the server is basically still TS3 and it wouldn't make a lot of sense to discontinue the corresponding client right away.
I wouldn't expect any feature updates though, only bug fixes and possibly removal of deprecated features (like they just did with rarely used codecs so they don't have to support them in TS5).