If you dump the tech file, you'll see there is a section called constraintGroups and within that there's a definition of various groups,probably including one called "foundry" which is the base level constraint group. This is how sets of process rules are organized in the technology.
The constraint system supports both built-in constraints and custom constraint types. In this context however, it was because in older versions that particular constraint (that was mentioned in the error message) was not implemented by Cadence using the native OpenAccess constraint (I can't quite remember why, possibly due to legacy issues with an older version still), and so was implemented as a custom constraint rather than a built-in one. When we migrated to use the new built-in constraint in OA, there was a discrepancy, and hence the warning.
I believe this is the case - I've not fully read up on it, because this change was a few years ago and I've forgotten the precise explanation (the first paragraph is definitely correct though).