SERVER-88630 confused several people until it was realized that the failure's cause was that enableTestCommands was disabled.

This ticket proposes that, instead of:

Unrecognized expression '$_testApiVersion'

... the server should give an error like:

Expression '$_testApiVersion' is for MongoDB's internal testing only.

This will deter external users appropriately, and folks who—like me —actually need to use it will know to ask in an appropriate forum, "how can I use internal-testing controls like $_testApiVersion?"

(The message could additionally actually say that enableTestCommands is the setting to enable, but that may make it too easy for users to hurt themselves.)