[2022] Mod to automatically destroy structures after inactivity?

Eco doesn't have that and I also don't know a mod for it. I personally also don't think that Eco should have that, it's quite a different game to Ark or Conan, I personally don't unclaim inactive stuff either if it's not just basic buildings, prefering to preserve peoples work that abandoned and getting the stuff that is really ugly cleaned up by government and auctioned off.

I get that, but I think the actual problem is the degrading performance to begin with, though I have not noticed any such degradation - the problems come from storage objects, not normal world objects as far as our investigations go. It's not out of question, but at least for now such a setting is not planned. (I noted it as suggestion, though)

That is something that is much more likely to happen and being investigated. The big problem with stuff just 'vanishing' after a while is also that it goes very much against of the Eco design goal of 'nothing simply vanishes' and it could be abused for a multitude of things. On the other hand, decay if we implement it will likely never cause something to 'vanish' either - even the last state of the object will still be in the world, just no longer giving any benefits and potentially the opposite, but not 'break down into nothing'.

The thing is that we don't see a reason to remove those to begin with (as I said, performance implications come from stockpiles and physics objects - for the latter there is a removal command, not from structures) - we do not recommend you to remove structures due to owners no longer playing, this is not Ark or Conan Exiles. And if you want to nontheless, we would want you to actually need to use work to do so and respect the laws of 'nothing simply vanishes'.

Adding a cheap option that makes stuff just erase itself not only violates the idea of everything in Eco being part of economy and needing labour and effort that can be paid it also makes stuff on that plot simply vanish and be removed from the economy. That not only affects buildings, but also things with negative environmentally impact.

Not to state that Rathelm's idea of decay and a abandoned plot looking ugly after a while without care by the state or it being repurposed or auctioned off could have implications on ecology as well that would be interesting to explore - especially for abandoned factories.

So, this has nothing to do with missing prudent thinking, we have specific reasons on why we don't do that. Of course anyone is free to make a mod for this option (and nid made one to automatically remove rubble), but there really isn't any kind of actual applicability at the moment, as it is very normal for Eco that servers have cycles that reset after a while where the problem you state is none and that the long term servers I know typically have the institutions that would allow for buildings to be removed by government OR the server not having a desire to remove such buildings, because one of the goals on long-term servers often is building cool buildings - and why would you remove great buildings just because the player left?

So you're presenting a very big edge case that even as itself doesn't really need such a feature as solution, there is other ways, we'd rather recommend and find to be within Eco's goals.