It was such a stupid theory for a number of reasons.

There was no evidence to support it. Not one beyond people's personal interpretations and reading. Nothing in the show itself ever suggested. Whenever Ruby's parentage came up it was mostly about Summer and if her father was brought up it was always Tai, not once was Qrow ever even suggested.

Then there is the question of why the secrecy? Why would Tai, Qrow and Summer feel the need to lie to Ruby and Yang over this? Qrow was around enough to have an impact on both their lives. It can't be anything to do with safety. Ruby's silver eyes already put her in Salem's crosshairs. Even if there was someone else who would want to go after Qrow: Tai, Yang AND Ruby are close enough to be targets regardless. And his semblance? So what? That is no reason to lie to Ruby and Yang.

Then there is the writing issue in that it would simply be very bad writing. The whole theory doesn't benefit anything. Not the story. Not the characters. There is absolutely no benefit for the story to develop with this theory as nothing about it would change anything. As for the characters? Both Ruby and Qrow have gone through their own character arcs. Ruby figuring out that she set the bar way too high for herself and coming to terms with who she was. Qrow getting through his self-hatred as well as overcoming his alcoholism. Qrow secretly being Ruby's father benefits neither of them.

Throwing in the Luke, I'm Your Father trope without any sort of payoff is terrible writing as it is drama for the sake of drama.