At least that long. The profession has a higher infestation rate of liberals than just about any other, probably going back to Dewey or further.
For many decades it's been clear that librarians, similar to Doctors and their Hippocratic Oath, are immensely proud of their own version - the Hypocritic Oath. As near as I can tell, it goes something like this:
In approx the mid-90's, Citizen Magazine, a conservative Christian monthly published by Focus on the Family, were getting their feet wet in the brand-new WWW, as we all were. They used the primitive web-search engines of the day (Yahoo, Altavista, etc) to see if their magazine was mentioned by anyone out there.
What they found was a heated discussion in a Usenet Newsgroup that was used by librarians to share info & tips with each other. The heated discussion wasn't over the merits of Citizen Magazine. Oh no, not at all. Rather, the discussion was over how to most effectively ban it from libraries.
It started off with a librarian complaining that a library patron had offered to donate to them a multi -year subscription to Citizen. She was asking fellow librarians across the country for advice on how to turn him down in a final decisive way that wouldn't escalate.
The first response was something like, "Just tell him you don't want his filthy, stinkin', right-wing magazine in your library!"
Others replied along the lines of, "No way! You can't do that. Some people might think it appears to be censorship of a political viewpoint." (Gosh! D'ya really think so??)
There were all kinds of alternate ideas brought up and discarded. Eg, "Well, sir, each periodical costs our library more than just the subscription, such as the costs of personnel to maintain the materials, etc." That would backfire, as the little Christian vermin might insist on increasing his donation to cover all handling costs too.
A better idea was to claim that their periodicals section was currently maxed out on space. But that lie could be easily uncovered by the patron perusing the stacks and noticing unused shelf space.
I think the favorite idea ended up being along the lines of, "smile broadly, thank them, flatter them, and then smother them with bureaucracy" by claiming that of course we will immediately forward your most generous offer to the Periodicals Committee, which meets periodically (and "periodically" == "never!" in your case, buddy) and please check back later to hear of any status updates. Eventually they'll give up, and you can keep your library pure and unadulterated.
Scuzzy. That’s what the profession has become.