If you specify SORT_IN_TEMPDB = on for an index CREATE or ALTER, then that CREATE or ALTER will sort in TempDB. If you do not specify it for an index CREATE or ALTER, then that CREATE or ALTER will not sort in TempDB.
The option is NOT persisted anywhere.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability