Proper 3u spaces with the proper 3u spacing for the stabs and proper 3u wires, for ALL of my boards. If you doing it properly its just as good as any of the other common sizes of space bars.

I probably have about 20 boards now that are 2x3u, its by far my preference for bottom row as I personally do not like a huge number of keys on the bottom row, its a complete waste for my workflow.

Most modern GMK Cherry and SA do not have the issue with availability and they ALL are the correct spacing. It has improved massively from when I first started getting boards with 2x3u, now you get 2x3u, its extremely rare to get 1x3u in modern GMK or SP kitting, Sure you usually have to buy the space bar kit so its not cheap, but then I have to buy the 40s kit most of the time anyway so...

If you expecting to pick up cheap sets from aliexpress or the like and have amazing 40s support complete with plenty of split space support, then its possible but highly unlikely. I have long moved passed the point I can be bothered with sets like that, I do not want to waste money on stuff I am bodging to fit as I just wont enjoy using it.

If the caps don't have good 40s support (which is usually limited to a few designers who hate 40s) then it likely wont have 3u bars, so they aren't getting my money anyway. I do have 40s with full sized mods and full size space bars so the option is there if I ever really want a set with shitty 40s support, but I rarely use those keyboards.