This shouldn't be legal. The software was clearly marketed as a classic fixed-in-time release, like the old CD releases, that would not be updated but would work indefinitely. Now they're going to boldly revoke the licenses???

It’s more malicious than that, they’re simply not renewing their code signing license hence making the software non-functional.

Legality has long since lost literally any meaning to megacorps and the ultra-wealthy pretty much everywhere. Doesn't matter what country they're breaking the laws in. The only penalty will be a paltry slap on the wrist fine (if even) and they will continue doing whatever they want. I mean, it's just the cost of doing business, isn't it?

It's definitely not legal in my country (Australia)

I don't know if illegal, but it can be breach of contract, microsoft can say "oopsie, sorry, our bad" or fight it in court.

They sold a perpetual product that broke in sync for every user, and the reason it is breaking is because of a license checking feature.

Not an easy case, but it could be argued they advertised a product as perpetual while it's effectively an X years license.

The fact that the breakage is related to the license might be relevant, you can stop supporting license checks, but do it to the benefit of users, not conveniently to their detriment as an upsale mechanism