For what it's worth, here's what Phil said about TAR 21 in his recent TV Guide Interview:
Do you have any updates on Season 21?
Keoghan: We start filming in the summer. We're working rapidly to get everything in line. We're excited to be going out again. Some stuff is locked and some stuff is up in the air. Up until the day we leave, we're still finessing. ... There'll be some new places and we're finalizing the cast. We hit a homerun with the cast this season. I think as the show has gone on, we've gotten better teams because more and more people who are fans want to be on the show.
He talks about filming in the summer. That would suggest June or later (technically June 20th to September 22nd), but I wouldn't put a lot of faith it that. I'd advise keeping our eyes open, but keep in mind that it could well be several weeks yet.