https://davidlat.substack.com/p/will-justice-breyer-retire-reading
David Lat reads the tea leaves on whether Justice Breyer is likely to retire this year, and concludes that he probably won't (based primarily on the fact that he has hired a full slate of clerks for next year). Such speculation is far from an exact science. But David knows more about the inner workings of the Court than almost any other commentator, and his speculation is more likely to be accurate than most.
A few brief additional thoughts:
It's still possible Breyer is on the fence (as Kennedy was in 2017-18, a point David knows). While it would be annoying to the would-be clerks if Breyer decided to retire at the last minute, they will surely still land on their feet, and probably get clerkships with other justices (perhaps even with Breyer's successor).
The liberal campaign to pressure to Breyer into resigning for the good of the cause doesn't seem to be working well. Such campaigns rarely do! Remember the failure of the similar effort to pressure RBG in 2013-14. Conservative efforts to get Alito and Thomas to resign last year (so Trump could fill their seats with younger conservatives), also failed miserably. With rare exceptions (e.g. - David Souter, who may be the only one in the last 40 years), modern SCOTUS justices like their jobs, don't find them onerous (scutwork can be delegated to clerks and other staff - and there's even a long summer vacation!), and don't like to be pressured. Many also tend to think that no potential replacement can possibly be as good at the job as they are.
Despite the above, I do think Breyer will resign either this year or next year, so Biden can fill the seat while the Democrats have a Senate majority (even if a very narrow one). The only caveat may be if it looks like the Dems will actually expand their majority in the 2022 election.
Of course, Breyer and I rarely agree. I even once wrote a highly critical review of one of his books (though I'm much more a fan of his earlier book Breaking the Vicious Circle). If Breyer finds out that I predicted his retirement, he might decide to stay on indefinitely just to prove me wrong!