Yes mixed fermentation and sour beers are very commonly served on draft in the US. The sourdough fermented beer that I helped brew is on draft in that brewery now.
I adore cask ales so it was a real treat to get to experience so many of them. They are hard to find here, and when something is labelled as a cask ale it's usually a beer that didnt go as planned and the brewery decided to add a bunch of adjuncts to it and sell it from a cask for the novelty. Think peanutbutter habenero chocolate stout...
I did have a couple up in Scotland that were not great and had very high acetaldyhide levels, and tasted like they were old and stale from low turnover, but that was the exception.
I much prefer a pale ale hand poured from a cask over a standard carbonation level keg ale.
My own beers at home tend to reflect traditional styles rather than trendy, or adjunct filled monstrosities as well, so I think that's just a reflection of my preferences in general.
Thanks for a very interesting answer. It feels like there is a huge fear of contamination of beer in the UK to the extent that any kind of mixed fermentation is deemed dangerous, and only specialist, small breweries do it. UK brewing historically involved brett and other things but science and progress removed it all!
The one thing we retained was cask ale, and that was under threat 50 years ago.
Our cask ales have the beauty of cask conditioning, which i prefer in standard ales to forced carbonation, but don't have the complexity of mixed fermentation. That's an acquired taste that was lost I think, and is still very much a niche thing. We do get a few sour beers in kegs in craft bars but you have to search for them or get lucky. They'll usually be simple Berliner types.
I mostly use standard English ale yeasts and Belgian/diastatic strains. I dabble occasionally with other things. I like bretted beers but only occasionally find them. I like some sours but some are too intense for me. My daughter says "the more sour the better." She literally sucked lemons as a 2 year old. Women seem more disposed to sour beers than men in my experience. Is that more widespread I wonder?
P.S. when Laĺlemand worked with Verdant in England to isolate their yeast and market it in dried form, they found three strains. The brewery had set out with one strain and picked two up. So that obviously happens. Breweries do use companies like Brewlab to test their yeast and supply new pitches etc though. Of course.