IntelliJ keeps a bunch of cache files in its "system" directory (settings and downloaded plugins are written to a separate config directory).
Is it safe to delete this directory (not while IntelliJ is running), without losing settings? The reason is that I might put it on a tmpfs, which gets cleared on shutdown.
It's perfectly safe though there are some quite useful caches there which will be lost. For example, you'll face long project reindexing, rebuild, Maven may decide to re-download the Internet, your VCS repository commits will be re-queried, the statistics used in code completion will be re-queried.