Movesets are based on the Gen 7 Ultra Sun/Ultra Moon learnability data. Though this is generally fine, particular HMs are blocked from being learned from a variety of Pokémon that should be eligible for learning it based on the trends of the HMs in previous generations. The most egregious of these is Rock Climb, which is one of the most commonly-use field moves in PMU (after Cut, Rock Smash, and Fly). It hasn’t been an HM since Gen 4, meaning there are three generations of Pokémon who have no access to it via HM — including those who can learn it through level up, such as Scraggy and Scrafty. As Rock Climb doesn’t seem to be one of those attacks that will be brought back as a field move, I believe that the “stay true to the official games” mentality falls short here, as I cannot think of any Pokémon who can learn a(n) TM/HM move naturally without being able to learn it through the TM or HM itself (except for Smeargle, of course). Additionally, this drastically lowers the availability of certain places until a needlessly small amount of Pokémon is recruited in order to access it.
I believe that certain TMs and HMs should be added to the learnability to the Pokémon who are in later generations than the TMs or HMs were released, at the least to better match up the type of distribution that the TMs and HMs originally had.