I was surprised to read (in the PDF on Schengen) that fingerprints are deleted once someone is granted residency. I wouldn't have thought governments would ever intentionally delete any data they collected, although they do tend to lose data quite often.

However, anecdotally when someone becomes a British citizen, their biometrics are no longer accessible from the database on which their biometric residency permit** data was stored. I don't know if they are truly deleted or if the record is just archived etc.

**I believe most people with ILR in the UK still don't have a BRP (and thus the government doesn't have their fingerprints) but they are also less likely to intend to become British.