I genuinely don't at all understand what the issue is with calling it a pagan library. It's a clear and substantive descriptor.
The history of the Roman Empire can be divided into two overarching periods - the pagan era and the Christian era. A pagan library is significantly more valuable than a Christian library, as we have far fewer pagan texts than Christian ones, and the few we do have were often corrupted by later Christian copyists. The word pagan is doing meaningful work in the sentence.
The word pagan has changed meaning, it would be more clear to call it an early Roman library. Pagan brings to mind a practice of worship in modern parlance.
It hasn't changed meaning, that's the only thing that it ever meant.* Religious practices are a hugely relevant distinction for the history of the Roman Empire, the same way it is relevant to distinguish pre-Islamic and Islamic Arabia. Early Christianity and Islam radically transformed the lives of the people subjected to them.
Just because the technical use of a term is not familiar to lay people doesn't mean it is unclear or unhelpful. Here on HN, we all know what C-style strings are, even if an ordinary person might hear that and picture some kind of nautical rope.
* In English, at least. The Latin word paganus did change meaning (the original sense was 'rural'), but this occurred centuries before the emergence of English.
The history of the Roman Empire can be divided into two overarching periods - the pagan era and the Christian era. A pagan library is significantly more valuable than a Christian library, as we have far fewer pagan texts than Christian ones, and the few we do have were often corrupted by later Christian copyists. The word pagan is doing meaningful work in the sentence.