Will the plugins - some provided by MS - still work as expected or will those be blocked or hobbled?
Thanks!
Nope. Only some.
So to use an extension in VSCode, it has to be published in Microsoft's store? And VSCode only in this other store?
You can always download the extension (or build it yourself) and install it manually - using Code or Codium. You can use the Open VSX registry with Code, but you have to configure it: https://github.com/eclipse/openvsx/wiki/Using-Open-VSX-in-VS.... So it technically does not have to be in MS' Marketplace, but 99% of Code user will not find or know of your extension, if it isn't in the Marketplace.
https://github.com/python-lsp/python-lsp-server
Someone might come along and tell you that you are missing out on some pylance features, but fuck closed source dev tooling it ain't worth it.
There was a ticket somewhere in the issue tracker with more information about this topic.
The plugins won't work out of the gate, but you can installed them manually and most of them will work.
It's not a 1:1 replacement, and from my limited experience, the UX of VSCodium was worse than that of VSCode. But, if you value not being tied to MS... That's the price you're gonna pay.