Traditional modding still exists, especially for PC RPGs, where the community is huge. Also UEFN is an extremely powerful tool that sits closer to modding Fortnite than UGC for Roblox, say.

The problem is that multiplayer games and any game with a server component - and the engines that make them - are specifically designed to have security against cheating and hacks. No-one wants to play against a modded game. That makes supporting mods, even solo mods, all the more difficult, and it’s harder to make the business case for doing so.