Honestly, I can't even think of too many instances in "current" 40K novels that necessitated a character to lock his bolter to his thigh. I'm sure that they're still in use, but as a specific detail to be used with the intent of telling a story, somewhat superfluous. While many technological advances have been lost, I would think that the Adeptus Mechanicus would have slight "pressure" to make sure the Astartes' battle plate was at least as good as it used to be.