Is necroposting seen as a problem here on PF? Screenshot below from another forum shows an approach to avoiding necroposts, at least those unintentional ones.
Necroposts can indicate a spam setup, so we do watch them pretty closely. They can indicate other problems, but sometimes they are just well-meaning attempts to help in an old thread. We check them all out, and deal with them accordingly. They stand out enough that I don't think we need any special signalling algorithm addition.
Plus, the PF members are great at flagging necroposts via the Report mechanism, so we don't miss many such issues.