These Legacy Template Variables (Release, Build, Execution, Promotion) are defined at the application-level, not at the system-level.
If you View Full Log, you should be able to find a block that looks something like this:
WARN : 2018-02-21 22:37:21Z - Legacy template variable declarations found:
WARN : 2018-02-21 22:37:21Z - ID: 107; Name: BUILD-SOURCE; Scope: Release
WARN : 2018-02-21 22:37:21Z - ID: 92; Name: BUILD-SOURCE; Scope: Release
WARN : 2018-02-21 22:37:21Z - ID: 108; Name: BUILD-SOURCE; Scope: Release
WARN : 2018-02-21 22:37:21Z - ID: 109; Name: BUILD-SOURCE; Scope: Release
WARN : 2018-02-21 22:37:21Z - ID: 111; Name: BUILD-SOURCE; Scope: Release
WARN : 2018-02-21 22:37:21Z - ID: 112; Name: BUILD-SOURCE; Scope: Release
WARN : 2018-02-21 22:37:21Z - ID: 113; Name: BUILD-SOURCE; Scope: Release
If that's not enough information to locate, you can look directly in the SQL Server Database to find out.