Thomas Rushton warns us that undocumented commands in SQL Server are liable to change without notice:
I don’t have every version available to test, but it does appear that the record indicating Containment State is mis-spelled in SQL Server 2012 as “dbi_ContianmentState”, and then corrected in SQL 2014 and later. It’s a good job I’m not relying on it for anything.
So, yes, don’t rely on undocumented functions – as they may change without notice.
If there’s a documented method, use that one. If the only method available is undocumented, you can still use it, but be sure to test it with each release—that is, major release, service pack, or cumulative update.