It doesn't matter what they choose. As long as they are accountable for the consequences of their choice.

And the company probably pays a platform team to offer a limited but supported tech stack.

The platform team is accountable for balancing the deprecation of outdated dependencies and the productivity of their users.

That's good actually. Groupthink and forced consistency are way worse.

Who do these teams report to? Either someone at a higher level instituted guidelines which allowed for inconsistent stacks, or someone at a lower level made the decision to disregard these guidelines and upper management took no action at the appropriate time to address the situation.

Either decision could easily be justifiable. The higher ups may have deliberately wanted to assure the teams were not stifled, or a team may have had a goal which demanded an exception to the rule. Still though, somebody had to have made the call and should therefore be able to justify it.

If an organization does not know who is responsible for something, then that is a failing at an even higher level.