pull-up is one of the few tags that is singular without a good reason.
Contrary we have most others in plural.
- Comparable tags like sit-ups and push-ups are plural.
- Unrelated tags like abdominals legs are plural, too.
- Only tags where plural doesn't make sense, like pain or bellyfat are singular.
Edit: I've just discovered that the chin-up tag is subject to the same issue. Most likely because it is nearly as hard.