As a special case: polyrepos are fine for when you have a genuine plugin architecture. To be a genuine one: you have a public facing end to end documentation on how to use it.

I would use chrome extensions vs. chrome itself as an example. Or same for VSCode. Then vendor plugins can be in their own repos.

I think otherwise for simple module A and module B that interact privately, in an adhoc way, the separation of concerns is not there and monorepo is better.