Ramifications of Full SYSAUX - Oracle What are the ramifications of having a full or almost nearly full SYSAUX tablespace (Oracle 19c)?
There are plenty of sites discussing how to view the SYSAUX occupants, to reduce the size of SYSAUX via dbms_stat.purge_stats(), to add datafiles to SYSAUX tablespace, and to alter_stats_history_retention to avoid the problem in the future.  This is not what is being asked here.
What are the consequences of not doing any of the previously mentioned tasks, but still utilizing the database?  I've read that SYSAUX is important, but not critical (critical stuff is in SYSTEM).  However, I have not seen any further discussion about what are the consequences of continuing to operate with a full SYSAUX.
For example, will performance be an issue?  If so, in what ways?
Does Oracle stop writing to SYSAUX or does Oracle try to delete just enough &quot;old&quot; items to make room for &quot;new&quot; items? (most likely it depends on what it is, but don't see any documentation or article that describes the resulting behavior of a full SYSAUX tablespace and what the resulting consequences may be and what to watch out for).
If one gathers stats will Oracle purge the oldest historical stats to make room for the new ones?  If so, is it true MMON gets called to do this &quot;on-the-fly&quot; rather than during the nightly process to maintain the retention setting.  If so, is this costly if an application is doing a lot of stat gathering and to what extent is it costly?
Thanks in advance.
EDIT: Please assume that AUDSYS schema is not in SYSAUX tablespace.
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.