Looking at a user profile one often finds this:
Apparently, this user prefers to keep an air of mystery about them.
This is terrible grammar. Them is only used in what might be considered a singular sense when no specific number is attached. For instance:
If anyone knows any reason why these two should not be wed, let them speak now or forever hold their peace.
It can be used thus because no number is assigned. Once a singular number is assigned, them is never appropriate.
I understand the current sociopolitical climate surrounding gender identity has made this a hot button topic but grammar like this is painful. Them is gender neutral but it is not number neutral.
As "this user" assigns singularity to the subject the usage of them is incorrect. Perhaps it could be reworded thus?
Apparently, this user prefers to maintain an air of mystery.