In addition to being a popular base for mixing, or just smoking on it's own, PS 80, 83, and 84 are popular for RYO. My own experience, FWIW, is that availability has been spotty over the last couple of years. Prior to that, you could buy truckloads of the stuff whenever you wanted.
Lower price, reasonably good quality, increased awareness of, hoarding, and perceived uncertain status in lieu of upcoming regulations are in my opinion driving the sporadic disappearances.
When you consider that most of the world's diminished tobacco production is locked up in contracts for cigarette manufacturers, the quantity of leftover product for the pipe tobacco market is somewhat static in the face of increased demand. Factor in the pandemic, and the reduction of manufacturing, and you get the dry spells.
That's my own personal take... I don't sit on the board of Scandinavian Tobacco Group, or whomever might own or be responsible for the production right now.