Are third party apps legal?

I don't see how they'd ban something made by Overwolf. Overwolf is a pretty highly reputable development company that works closely with companies to ensure they're adhering to EULA.

I'm 95% sure anything made by Overwolf is safe.

If they made something ban-worthy, that would tarnish their pretty stellar reputation -- and that's bad for business.

This is kinda like the Blitz.gg thing yeah? I have no idea, I've seen people about getting banned for it(whether or not they tell the truth, idk) and others who say they've used for ages and have been fine.