These days, when a sequel to a simulator comes out, it’s almost invariably lacking options compared to whatever came before it. With the advent of DLC (and even before that with expansions), simulators often stand as a platform for more content. So, for retention’s sake, do the existing City: Skylines mods work in Cities: Skylines 2.
No, and I think that would be a rather strange expectation.
On one hand, it’s true that both games are built in the Unity engine, but that doesn’t really mean much. All the systems underlying the game have been reworked, so any reference a mod makes to existing facets would simply break. Unless a game is simply retitling itself for a big update, mods transitioning to a sequel doesn’t really happen.
On the other hand, modders who created assets for the original Cities: Skylines will be able to reuse their old models in the new editor because they’re made separately from the game before being imported. That might make for quicker porting of older mods, but it’s still a manual process.
Furthermore, modifications aren’t even officially supported by Cities: Skylines 2 yet. When the feature is available, it will be using a system called Paradox Mods, rather than going through Steam Workshop or Nexus (though, realistically, external Nexus mods will still happen). The purpose of Paradox Mods is to allow alterations made on PC to carry over to the future console versions of the game, enabling cross-platform.
So, to reiterate; no. Cities: Skylines mods will not work in Cities: Skylines 2.
