Broadly/generally, many people seem to agree that there is a very short progression system in Unturned 3. Personally, I believe the only map that does loot progression “well” is Russia.
Nelson has openly acknowledged that Unturned 3 has both a hardcore and casual playerbase, and that adding one thing often ends up getting negative feedback from the other. 4 has been openly stated to be heading towards a “darker” more hardcore direction, which includes not encouraging PvP through end-game items (and guns in general) spawning super frequently. Fixing loot progression is high on that list, especially so PvE has a place in the game.
That’s only a small fragment of the reason for a new game though.
Anyways, iirc (without checking) he’s never directly said anywhere directly that he plans to straight-up decrease item spawns, but he does plan to “balance” the value of individual items better. This includes making things like melee viable again, and fixing PvP/bases. He’s also stated somewhere that he plans to better balance the Inventory system[1], and that he plans to expand upon Unturned 3’s game mechanics to be more challenging, skill-based, and/or rewarding.
Specific changes in Unturned 3 in the past that were done as compromises to try and move towards a more hardcore feeling most notably include making guns and ammunition spawn less commonly naturally, and adding two more weapon calibers (the high calibers).
So yeah, some of it probably is an assumption, but it’s a fairly good assumption to make based on Nelson’s current (publicly stated) vision.