* For an example of "Drop B", at one point the haskell-lsp package was deprecated in favor of the lsp package, so all downstream packages had to drop the haskell-lsp package as a dependency and migrate to the lsp package (I personally had to do this)
* For an example of "Don't let A change", that might happen for some period of time, although not indefinitely. The most obvious example is holding back the compiler version. For example, Stackage was on GHC 8.10 for a while, even after GHC 9.2 was released, due to breakage introduced in GHC 9.0.
* For an example of "Ignored B's declared dependency constraints", this is extremely common, especially when the `base` package is upgraded (since many packages have conservative upper bounds on their `base` dependency which can often be trivially bumped without issues).
* For an example of "Take over development of B" the `aeson` package for JSON support is one example of this. More generally, this happens when abandoned packages get adopted by the Haskell organization.
And Stackage does require contactable maintainers for supported packages. There are some exceptions to this rule, though. For example, sometimes a package gets added where the maintainer is available, but a dependency for that package was not yet on Stackage. I believe you can either add that maintainer to be the contact for that dependency, too, or it can be an orphan package. There are a bunch of people who fill in the maintenance gaps in the ecosystem by fixing these packages that don't have official contacts or active maintainers.