I changed it to say "Launch YC" instead of "Launch HN" this time round, for reasons related to what you're saying. Doesn't seem to have landed though :)

Another thought I had was "Meet S21". Better? It would free up a char! "Meet YC S21" is too long.

"YC S21" would be the shortest. That is, something like "YC S21: Foo, Bar, Baz" with a word or phrase for each startup. But I'm not sure if that would convey that it's a launch thread.

Another possibility: "YC Launch" instead of "Launch YC".

Our original idea was "Meet the Batch" but that's way too long.