They are not only backward compatible or bug compatible. They are others-person-bug compatible. It's the only way to prevent users thinking about switching to another OS.
https://www.newsbytesapp.com/news/science/before-affecting-w...
But I don't think Microsoft verifies customer code, they might not even have access to it.
There is a recent video now from a former Microsoft employee where he explains that those drivers that get WHQL certification are ran on test machines in stress conditions for some time, or at least that is how it used to be when he worked there.
Since that process is probably quite slow to be able to push update within a couple hours Crowdstrike just bypassed the QA testing by injecting their own data files into the driver.
It’s not meant to signify that it’s bug-free.
I don't think Microsoft is realistically in a position to forbid other companies from writing kernel level modules, from an antitrust standpoint I would think that would land them under investigation(s)