I think there would be a need for some tag_group like this, where one could easily find all sorts of stocks, breeding_stand, arms_tied, collar, frogtied, pinned, restrained, bound, etc tags. But which title?

You seem to want to group bondage related tags, so wouldn't that be better option than bdsm which also contains stuff like power play and other roleplay stuff which has nothing to do with what you want to organize here.

if you're going for an inclusive tag_group bdsm would be the higher level one since bondage is implicated to bdsm, for just the tags you mentioned you could probably go with bondage. not sure you'd need a tag_group though, you could probably just expand the wikis for bdsm, bondage, and bound and then the tag wiki's themselves would fulfill the need for a tag_group.

Thanks to both of you! Indeed the bondage page already has a good beginning, as well as those two other pages. This also clarified my understanding of the purpose of the tag_group mechanism.