It was a switch in the original wallet code allowing quick exit by bypassing clean shutdown and, if memory serves, also saving the blockchain (which was pretty lengthy).
This mode was not maintained since (1) using a database made saving instantaneous since the database is now maintained on disk, rather than in RAM, and (2) miscellaneous fixes and improvements were made to the code in general to remove/shorten some blocking operations.
The fast exit code was not reachable anymore, and I'm not aware of any specific issues it caused. It just wasn't needed anymore, and so could be jettisoned.
Since it was not reachable, it is very unlikely any new issues were introduced.