This is not good, and I see that DROP SYSTEM VERSIONING also removes these columns which is even worse. Please read the standard that you are trying to implement!
The standard states the function of ALTER TABLE ADD SYSTEM VERSIONING
as "Alter a regular persistent base table to a system-versioned table" and
system versioned table is described in the standard by two generated
stored constraint columns and implemented as such.
I will do a more thorough review of the functionalities in this patch (not necessarily the code) this week.
