Yes, questions asking about online resources are on topic. There is nothing that makes them off topic; contrary to the tag wiki they are not categorically and inherently about recommendations. The tag wiki doesn't accurately reflect usage of the tag, and is from the year before game recommendations were ruled off topic; I'm updating it (see end of post).
They contain a lot of recommendations, but that's just because it's one of the things people asked for recommendations of. The fact that online-resources contains a large proportion of a no-longer-accepted question category does not make it synonymous with that category.
Concerning those last two, let's recall that a definitive part of recommendations is that we are bringing to the asker a variety of suggestions for things they could use in hopes of finding a best-fitting one. They ran afoul of "every answer is equally valid". Here, providing what is factually the canonical resource is not a recommendation of something equally valid, it is the correct answer. These are like content-identification or product-identification requests. An answer that says "use this" is not, inherently, a recommendation.
I've updated the tag wiki to the following:
for questions about using or finding online resources. (Recommendations of "best" resources are not on topic, but we can provide guidance around identifying canonical resources.)
This tag is for questions around using online resources or identifying canonical resources.
Recommendations for game materials are now off topic, but we can provide guidance around the existing canonical resources available.
Related tags are: online-roleplaying, play-by-post, or tools.