Last year, Journeyman Geek asked this question about where Stack Overflow Teams questions belong. I tend to agree with most of Catija's reasoning at the time
Teams is still predominantly a Stack Overflow product - Teams are hosted on SO and the only way they reach out to the network is in the left navigation. For the most part, it still feels to me that Teams questions should remain on SO for the time being.
The problem is that part about "predominantly a Stack Overflow product" is no longer true (per staff)
To get to the meat of your question "Why is the removal of Teams from stackoverflow.com necessitating the removal of all ties?", we’re currently decoupling Stack Overflow for Teams from stackoverflow.com as part of a larger project to achieve SOC 2 Type 2 and ISO 27001 certification in the future. I can’t go into further detail at this time (a more detailed Meta post is coming later next year). This also helps Stack Overflow better service, test, and improve Teams without making infrastructure changes to our public site (and vice-versa),
To put it another way, SOT is now fully divorced from Stack Overflow. It no longer appears in any navigation (save for the "Looking for your Teams? " link in the network-wide side bar). That modal now says
Stack Overflow for Teams has its own domain!
You can now access your Teams at stackoverflowteams.com. Teams no longer appear in the left sidebar on stackoverflow.com. Check your email to learn more about these changes.
The only "Stack Overflow" part that remains is now the name. Should we continue to migrate questions like this to Meta Stack Overflow?