The sane choice would be to not engage reverse thrust at all (until the pilot has reset the reverse thrust throttle) or to only engage reverse thrust once the landing gear has weight on it.
Having reverse thrust depend on a sensor that could fail seems like a poor choice. What if the landing gear don't drop? Is there a situation where you would still want reverse thrust without landing gear?
Actually, maybe some ex-Soviet airliners can still do it. DC-8s being used by cargo airlines could possibly still technically do it, but don't use the ability.
> What if the landing gear don't drop?
I don't think that there is need for reverse thrust in such a scenario.
> Is there a situation where you would still want reverse thrust without landing gear?
Even if there is, there could be a manual override.
How do we slow down planes that still need to come down despite a stuck landing gear, then?
Indeed, a manual override must exist.
It's also a sensor that you can simply put on each wheel to determine if there is weight applied.
Or if the manual override is turned on.