libreoffice should please respect folder standard. there is .cache, .local, .config. currently libreoffice writes: .config/libreoffice/4/user/uno_packages/cache/
I am unsure, but my suspicion is that this is NOTABUG, and our "cache" is semantically different from what Linux considers a cache. I actually suppose that flushing our "cache" could break the extensions - or am I wrong?
Pruning the ~/.config/libreoffice/4/user/uno_packages/cache/ directory would mean that per-user installed extensions are lost. Just that something happens to be (mis-?)named "cache" doesn't necessarily imply that it would best live under XDG_CACHE_HOME.