EssentialsX have 'max-fly-speed' feature (range from 0.1 to 1.0 (default flySpeed is 1.0 ??))
But looks like this work only when use fly by command, and don't work, if player fly by WGFlyFlag, for example.
Can you make this restrict global? And can you deny fly exeleration for players, which don't have special permission, when this restriction active ??

I believe this option only applies to /speed rather than /fly, and so it won't apply to other plugins. I don't think that's something we're intending to add, because we'd need to somehow support individual plugins for this to work.