MacOS 12 Monterey is not out yet — and, I hope, won’t ship until after a few more weeks of much-needed bug fixes — but Safari 15 for MacOS 11 Big Sur shipped last week. I strongly recommend not upgrading, unless you’ve already tried the new tab design and like it, or at least feel ambivalent about it. Updates like this are why I always turn off “Automatically keep my Mac up to date” in System Preferences → Software Update… More here.
Good advice from John here. This is a Marmite change that you will either accept or hate, I can’t see ‘love’ being a likelihood.