As I replied to you on Discord, the variable was never documented, and it will work in the next update. Its usage is not that different from the FSDT_Frozen variable, but there are some differences:
- The FSDT_Frozen will tell airplane developers the moment GSX froze the airplane, so they can use it to disable any kind of custom systems they might have in place that might be affected by it which would end up conflicting with GSX moving the airplane while frozen. Note that, a developer could have checked even the standard FREEZE SimVars as well, but there are several of them, this one is just one variable, so it might be easier to check, even more now that L: variables can be checked directly with Simconnect without using a WASM module in between.
- The FSDT_GSX_BYPASS_PIN is similar, but is tied to the precise moment the bypass pin is inserted/removed, which might not always happening at the same time as the freeze of the airplane, so it might be more useful to precisely turn off systems specifically related to the steering.