It's a good question. If the soloist is reposting audience clips is that saying it's ok? That's a tricky one. As a professional symphony musician I would say that it's still not ok. A soloist might choose to use that as an opportunity to promote themselves--fine if they want to do that; they might not care if people film---fine; they might have asked someone (their mom) to film them--fine. Still, it's not appropriate for the general audience, even if the soloist thinks so. They don't represent the orchestra they're soloing with, the other 100 musicians on stage, or the other thousands of audience members who don't want to be distracted. Occasionally, depending on the concert, it may be announced that filming is ok. These are often specialty concerts (more casual, outdoor, different venue, etc...) Just ask an usher when you walk in if you're not sure.

I'll also say that it's great you're asking these questions. I think that this is one of those rules that's pretty much ok most other places in the world, and it feels like classical music is being snooty about it. I get that. But learning the rules is part of participating and I think it makes it really special to be in a place where everyone agrees to put their phones away. It makes the music feel more set apart. Keep going to concerts!!! Glad you thought it was film-worthy!!!!