I generally use the singular for uncountable nouns (no choice anyway) and for general concepts that generally arise in a single instance, and the plural for countable objects when there are generally potentially many such objects. As a rule of thumb, would you say that the question is
As you can see from the examples, there's quite a bit of ambiguity.
Some tags have to be singular (uncountable nouns), so we can't systematically use a plural. Other tags don't really work in the singular (e.g. file — you never work with one file, always with files, plus file to a Unix person looks like it's about the
file utility), so we can't systematically use the singular either. No Stack Exchange site that I've seen has established any kind of consistent rule. Security comes closest to my personal preferences even though I wasn't behind the tag names; it has:
- encryption, web-application, authentication, cryptography, network, hash, web-browser, malware, email, privacy, penetration-test, …
- passwords, certificates, attacks, firewalls, …
We shouldn't try to enforce a singular-only policy, as it would force some bad tag names. Let tags arise naturally in either form, and rename if we find a strong argument to rename.