Oracle SYS views missing from DB instance
No, the database will not drop its own views.

But I have seen this many times, for example when developers thought naming their own tables as 'T$...' and views as 'V$...' was a good idea, then they wanted to clean up their schema, but they messed up all the V$ synonyms and views.

I would just start the database in upgrade mode and run the below scripts to rebuild these objects:

@?/rdbms/admin/catalog.sql
@?/rdbms/admin/catproc.sql
@?/rdbms/admin/utlrp.sql


Run Scripts to Build Data Dictionary Views