Is it okay to ask for algorithms (or libraries) on this site, or is only software (as in specific implementations of algorithms) allowed?
If not, is there another SE site that invites such questions?
Asking for libraries is part of the scope of this site — we field all software recommendation questions (in the sense of “what software can solve my problem?”), whether the software is intended for end users or for professionals (such as developers).
Asking for algorithms is another matter. Algorithms and software call for different expertise, and invite different kinds of answers. Questions would have some aspects in common — explain what problem you're trying to solve, state precise requirements, etc. — but these aspects also come up in recommendations for cars, recipes, career paths, etc. all of which have their specific expertise and specific difficulties. I don't see how algorithm recommendations would be on-topic here — answers and reviews would come from computer scientists or engineers with a strong CS background, not by IT professionals.
“I have this computational problem and I'm looking for an algorithm to solve it” falls into the realm of applied computer science. As it happens, Stack Exchange has a Computer Science site where such questions are on-topic. (As it happens, I am a moderator both here and there.) CS.SE doesn't have an official set of guidelines for algorithm recommendation questions, but usual advice and guidelines apply: describe the problem you're trying to solve (What do you need to compute? What properties does your data have? What order of magnitude of data size, processing power, etc. do you have? What are your performance and interoperability constraints? …). Once you've established what algorithm(s) are suitable for your problem, you can ask here to find an implementation that would fit in your development context.
I do believe libraries are, assuming the question meets quality standards.
Algorithms, in my opinion less so. They're a how to solve a problem (more what you'd ask on the trilogy [Stack Overflow, Super User, or Server Fault]) than a what you use to solve a problem (what we primarily deal with here). You can stick a library in your code and bob's your uncle. To properly use a algorithm, hopefully you would need to understand how it fits in with your program, and all manner of other considerations
It would very heavily depend on how you ask the question but it feels like scope creep to me.