To clarify: The possibility to install 3rd party apps wasn't removed in v5!
Plugins were deprecated, but applications can still be created, exported, imported, etc..
Hooks were replaced with Code Listeners and several new Extensions
But all in all, 3rd parties will still be able to create new apps like Member Garage, Member Maps, Classified Ads, Member Info blocks etc..
More information about the upcoming changes can be found in our Dev Blog: https://invisioncommunity.com/developers/devblog/blog/