These comments are my opinion and are not suggested as either the only or right way to consider the question. The library provides access to constitutionally protected speech. This is a question that must be asked of each potential vendor. To my knowledge, no filter claims that it only excludes constitutionally unprotected speech. The technology and techniques used by companies that produce filters are improving.
Many filters can be fine-tuned.
An example of such filter fine-tuning would be one that excludes sexually oriented nudity. Sexually oriented nudity, however, is not necessarily legally obscene. It depends on what policy the library's governing body uses to make the decision to offer access. The content of the Internet is continually changing. Such change makes it extremely difficult, or impossible to make a selection decision for each individual site.