Comment by nybble41

The FHS is not wrong; cleaning policies are indeed site-specific and files placed in any temp directory can in principle disappear at any time. (Though, in theory, it's not supposed to happen while the files are still "in use" by running programs.) Still, historically you could count on files in /var/tmp lasting longer than files in /tmp, including across reboots.

Nothing will immediately break because you linked /var/tmp to /tmp. Whether it causes issues depends on the programs that you (or your users) run and how they make use of /var/tmp. However, if someone did have to restart a long-running process from the beginning because recent state information in /var/tmp was not preserved across a reset, I would say that is a problem with the administration of the system and not the program that stored its state there.