Sure there are. Doctors and actors, to name just a couple. In both cases the "union" actively works to create barriers to entry.
The AMA colludes with medical schools to set artificially-low student body quotas. If you've ever wondered why teaching "XYZ for pre-meds" is such a miserable experience, this is why. You have to earn straight A's to get into med school because there are so many more qualified candidates than openings (but it's not clear to me how, say, art history or algebra-based physics makes you a better doctor).
SAG (the screen actors guild) requires actors to have already performed in a SAG production a a condition of membership. And they also strictly limit the number of non-SAG performers on SAG productions. That chicken-and-egg problem was very intentional
If you've ever taken a macro economics course, you know what effect these actions have on prices.
> I'm not aware of any evidence for markets where demand outstrips supply (like that for skilled software engineers). It's not immediately clear that union protections would be beneficial.
See above. Unions can create a market where demand outstrips supply.
> If unions are better for workers, why is it that a non-union area !!with a cartel depressing wages!! was still substantially better for workers than a unioned area with no such issue? Saying "oh the market is different" ignores the question of why the market is different.
So tell me why professional associations exist, then. Why do doctors form a union to increase wages, if as you say, they would be better off without it?