I clearly remember that in H19~H20.5, you can select multiple nodes and just press R repeatedly to switch the display flag between them.

But somehow it doesn't work in H21? How to quickly switch flags between nodes now? I don't even know what such a useful QoL feature just got... removed?

Interesting, I didn't know about this feature before. I checked just now. Technically, the R key switches to another node, but it resets the selection after switching, so you have to reselect several nodes. Technically, the flag is toggled. So the problem here is likely that the selection of multiple nodes is reset.
AI made me a script that sets the flag on the closest node (implementations of this were already on the forum), but then switches the last two nodes in a circle with the same hotkey, which is very convenient.