Re: Strongly discouraged
Let's take the PS5 as an example here. The entire bus is optimised for a particular SSD. If you change it then the optimised operations will be slightly off. It may still work but it won't work as well. I'm guessing that MS is implying the same thing. Why else would they discourage you buying their own up-rated SSDs?
Perhaps I'm being naïve but these top-end systems often have very precise tailoring for the exact spec they are shipped with. Plus I was fascinated by the lengths that Sony went to in tailoring the new PS5 storage sub-system. I might buy one for Christmas, just don't tell my XBox-One.