Using 2.81, it looks like you can now power off a line split, but it doesn't appear to have any effect on the split.
In my case, I've got a split and on both the A and B sides I have a few different effects on them. I use Exp 2 to slide the mix between the A and B lines. When I power off the split, I was expecting that Exp 2 and all of the effects on line B would be disabled because powering off the split has effectively removed the split from the mix.
I've attached the preset. Use the "Fuzzy Roll" snapshot and roll Exp 2 with the split's power on and off to see what I mean.