It seems the stationary_restraints tag has spread to pixiv, but was assigned as a translation for 四肢拘束, which at first glance seems to indicate general restraint, but 四肢 seems to indicate it's specific to when all four limbs are restrained.
I've been poking around pixiv's Crowdin, but I haven't figured out how to suggest a correction, if there is a way to.
In any case, it seems to be resulting in the tag being applied to unfitting images here. stationary_restraints is about restraints that are rigid and stationary, like mounted pillories or a glory wall, in contrast to things like a length of rope or handcuffs that will move along with the subject's struggling.
Updated