Just to add to this, myself and my brother recently tried multiplayer with the
F18 and all functions including landing gear were working correctly and
independently of one another which was not the case previously. Does anyone
know if anything has recently changed?
Bumping this idea again since it is a new year, hoping to get more votes from
the community to see this happening at some stage 
Best, Raul
Having lived with this issue as MSFS has evolved since 2020, IMHO it’s best
treated as two completely separate issues: (1) the fact that L:vars are shared
across all aircraft in your local sim. The issue for me ultimately wasn’t that
this was happening, it was that it was undocumented. I was able to keep L:vars
just for sounds, and migrate everything else from L: to Z: and O: vars. I
still make extensive temporary use of L: vars to benefit from the inspection
in the Behaviours window. (2) the need for vars propagated across multiplayer.
Again this was particularly an issue because the current status wasn’t
documented, but we’ve subsequently discovered multiplayer communication is
limited to indexed external lights and certain external controls animations
simvars. E.g. (LIGHT NAV:1, bool) will be communicated from the user plane to
the remote AI multiplayer slave. So the main issue that affects our
multiplayer gliding is (2), and we’ve enthusiastically repurposed all the
exterior lights simvars for animations that are important for us in
multiplayer (ballast dumping is the best example). Of course we periodically
bump into some other mod which is messing with the same lights. The best
solution for us would be a defined max for data size/frequency (I understand
multiplayer comms always has a budget) but the SimObject could define that as
a STATIC (Z: name) DICTIONARY, i.e. that could be defined a in a SimObject
.cfg file, instead of requiring everything passed across multiplayer being
hard-coded into the sim. A smart design would not need to transmit the names,
as those are already available at both ends.
And Now SU14 has killed all that, so back to square one !!
A lot of Devs are scratching their heads now, trying to figure out another way for their plane to work correctly again over MP, with custom dynamic modeling.