Ramifications of Full SYSAUX - Oracle
What are the ramifications of having a full or almost nearly full SYSAUX tablespace (Oracle 19c)?

Nearly full?  None at all.
Completely full?  Probably a dead database!
By far the biggest consumer of SYSAUX in 19c is the Unified Audit Trail.
Where we used to dump gigabytes of Audit files into a file system somewhere and delete them occasionally, 19c now insists that we dump the same gigabytes of Audit data into tables held in SYSAUX, by default.
It has always been the case that if the Audit system cannot record what it wants to then it throws an almighty strop and crashes the entire database.
Nothing like subtlety.
So, if your Unified Audit Trail is writing to SYSAUX and SYSAUX fills up completely?
Bye bye, Database.
Oracle recommend changing the tablespace used by the Unified Audit Trail.
Kind of makes you wonder why they don't ship Oracle Database with a separate tablespace for this in the first place.