I find this language very confusing:

This seems incomplete as a policy: how are readers supposed to know which subtypes are semi-mandatory? I'm fine with the provided examples being semi-mandatory, but they are not presented as an exhaustive list ("include but not limited to the following").

Maybe we should change it to "the following are strongly recommended in languages that have the relevant construction", and consider adding a few more (advcl:relcl? nmod:poss?).

There are some subtypes used in a lot of languages that may reflect legacy relations we don't necessarily want to recommend—e.g. flat:name, flat:foreign, obl:tmod.