Or has the developer stopped working on it after the tragic fate of openIV?

Maybe you don't have those files. Check GTAV/update/x64/dlcpacks/ and see if there's the dlc file, if not you can download one online

I downloaded an add on vehicle, which worked and could spawn in the past but now it doesn't spawn. I do not have other mods except for trainers used to spawn the vehicle and the only one add on vehicle. Any solutions for this? I've tried gameconfig and it still doesn't work, because the problem is not its limited It should not be the vehicles problem because as I said it worked before but not now.