How "legacy" is the legacy gauge or FX stuff, actually?

In the latest SDK, for instance several TACAN-related #defines were added to
WASM/include/MSFS/Legacy/gauges.h . Also, the F/A-18E uses a new (?) “legacy”
.fx for its afterburner. Does this mean that such things, though said to be
“legacy,” are actually still fine to be used? (But others, like the legacy
flight model, probably truly are not to be used if you at all can avoid it.)

F/A-18E uses a new (?) “legacy” .fx for its afterburner No it does not. New
type of effects used - since last update effects editor got support of
luminosity (for flame glowing) and heat blur. Some legacy FX files no longer
works already, all of them will be replaced completely soon. As for
mentioned "gauges.h " it contain functions and definitions for DLL gauges
source code support. Can’t say for sure but I think it will be available for
a long time as some of them still useful.

F/A-18E uses a new (?) “legacy” .fx for its afterburner > No it does not
So the LIGHT_ASOBO_Afterburner.fx is truly “legacy” and did exist earlier
already? It is mentioned in the asobo-aircraft-
fa18e/SimObjects/Airplanes/Asobo_FA18E/systems.cfg. (I don’t have any copy
of the pre-SU7 installation to compare to, and sadly for some reason Windows
has updated its timestamps even though I only looked at it in Notepad, so I
can’t say what its timestamp was right after installing SU7. But most of the
.fx files in fs-base-effects-legacy/effects have timestamps from December
2020, but some are from the second half of this year. Is that not a sign
that new .fx files are still introduced?)

Ah this one. That is light, not very same type of effect as smoke. Modern
light system not exists (yet) so you are right - they still in the action and
has no alternative. They are legacy in the meaning that was migrated from
previous version of the sim but not abandoned. Effect itself (flame and glare)
defined in model/F18E.xml as ASOBO_FX_AFTERBURNER template, but modern effect
can’t insert light source into the scene so FX light used for that.

Ah thanks, that makes things much clearer.