Originally posted by Freedon Nadd*Legends. There's not much reason to think Sithisis itself is canon, just because the Sith shrine beneath the Jedi Temple is canon.
Sithisis is non-Legends, then?
However, even IF Sithisis were canon, I'd sooner think Palpatine was preforming said rituals at his base in the LiMerge Building on Coruscant, as opposed to underneath the Jedi Temple itself.
After all, we know for a fact that Palpatine canonically used the LiMerge Building to preform other, very powerful, Sith rituals/magic during TCW, so it would make sense for the rituals in Sithisis to take place there as well. Trying to preform rituals of that magnitude in a shrine just below the Jedi Temple would have been a bit too bold(even for Palpatine), and undoubtedly risked drawing attention from the hundreds of Jedi above. /shrug