There's a difference between it being "available" and including it as a default repository in sources.list.d. Available means it's a choice. Not everyone wants MS software on their computers. I don't. This means that your systems checks in with MS every time an update is run. If others follow suit out of a "Me, too!" obligation, I may lean to NetBSD or OpenBSD on my Pis. I severely dislike MS's telemetry and how they have their hooks in everything. My software should never phone home unless I want it to do so.

It was available "unofficially" from here:

https://github.com/headmelted/codebuilds

Looking here: https://packages.microsoft.com/repos/code/pool/main/c/code/ it seems, it has been available natively for some time already.