rfsctl is no longer user accessible (shell is not available to users either), and you will need to issue commands as documented in the AltaVault CLI guide. For setting the eviction threshold, you should issue the 'datastore eviction threshold' command (default value is 90%). Alarms that can be adjusted are displayed in the alarms page in the UI (Configure > Alarms).
I'm able to modify eviction threshold (victer.maxpctused) using CLI, but I'm wondering if when setting the evictor threshold to arbitrary values we should also adjust evicter.alarmwindow in order to keep equation balanced:
The rfsctl is not available beyond support to modify any other settings on AltaVault, and therefore you do not have access to configure any other evictor settings beyond the one already discussed (setting the eviction %).