Most of them were ad-less free apps that offered authenticated access to free APIs, so pure costs to Twitter, but Twitter hadn't been able to shut them down due to feature deficiencies in official apps.

I can confirm at least some of the free apis are still in working order. Twitter started to throw an error yesterday afternoon that my app. wasn’t in a project but I was able to resolve it.

If there were rev sharing / paid you'd be hearing about lawsuits already. Lack of lawsuits tells me the freebie got cut off.