Are photography contests on Threads worth it?

It really depends on what you're expecting. If you're looking to make money or win big prizes, probably not, because most contests on social media have small prizes or are more about visibility. Now, if what you want is to showcase your work, connect with other photographers, and get feedback, then yeah, it can be worth it - especially if the contest is well-organized and has judges who actually know what they're talking about.