This is used by .Net core [1]. .Net core checks the presence of CONTEXT_EXCEPTION_REPORTING flag and refuses to "hijack" its threads during suspend without this flag (then checking before _EXCEPTION_ACTIVE, _SERVICE_ACTIVE if it is present). That is even on x64. Without this implementation I so far didn't spot it fatally breaking but at least it does more SuspendThread / ResumeThread without that implemented.
I think originally these flags were used for WoW64 where Windows (and Wine in the new wow64 mode) can end up with broken context from NtGetContextThread() if the thread was suspended in an unfortunate time (during transition to 64 bit syscall or back to the userspace or delivering exception). [2] is another example of using it. I don't know which apps are using that specific garbage collector, but probably this functionality might be used even without .Net core.
1. https://github.com/dotnet/corert/blob/c6af4cfc8b625851b91823d9be746c4f7abdc6... 2. https://github.com/ivmai/bdwgc/blob/4d6cdc072663c1a22379d84acdac8a273ace1bc9...