i've commented tests in ckan/tests/legacy/functional/api/model/test_relationships.py as im not sure if they should be rewritten or just deleted

I think that relationships worth rewriting - we still have this feature, but it wasn't updated for a long time(half a year ago i fixed one bug and everyone was surprised by the fact, that there is a person, who try this functionality:))

I was curious about why this tests were still passing even with the actual removal of the rest API and turns out that they were never run in the first place 🤷‍♂️ The test classes don't start with Test so they were not pick up by nose.

@smotornyuk regarding the relationships tests I think you are right, they should be rewritten if we keep the feature (see #4212), but looking as they are not run and we can use history to get them if we ever rewrite them I'd just remove them for now.

@tino097 can you remove the commented lines and merge into 2.8 once this is merged into master?