While I can’t say I recall having read any actual journal articles on the matter, I have read a number of different things on it and reasonings behind certain limitations.
What limited and possibly (in)accurate information I know is this. At least for indoor rabbits who may have limited or no access to sunlight, calcium may be a larger issue than just sludge. Vitamin D aids in calcium absorption, so a lack of vitamin D and insufficient calcium greens in house rabbits could potentially lead to dental issues or brittle bones (deductive speculation on my part) due to insufficient calcium in the diet. In addition to that, without vitamin D and with a high calcium green diet, a house rabbit may have too much calcium passing through their system and ending up in the bladder because the body can’t absorb it properly. Again, more speculation.
On the other hand, I do know that a rabbit’s body is designed to excrete excess calcium through the bladder, it’s just when this calcium becomes in excess that it is an issue. I also remember reading something about rabbits being more susceptible to sludge are those that aren’t as active, or may be stuck in smaller cages. Because rather than the calcium passing evenly in the rabbit’s urine, it collects at the bottom of the bladder as it’s not being stirred up as the rabbit is hopping around and doing binkies and running etc. Can’t remember where I read that, but I remember thinking it was an interesting idea at the time.
Thirdly, I believe when talking about dietary calcium, you also need to consider high oxalic acid greens. As oxalic acid limits the uptake of certain minerals etc. and I believe actually binds to calcium. So if you’re feeding lots of high calcium greens, and lots of high oxalic acid greens, then that is potentially when crystals will be forming because the oxalic acid binds to the calcium to form them. At least that’s what I remember anyway.
All that being said, my aim for leafy greens was always to alternate days. One day would be my low oxalic acid day, the next was my low calcium day.
All that being said, you are probably correct that there isn’t a need to limit calcium unless the rabbit in question has a history of sludge. Just the same with people getting bladder stones I suppose.
Take what I’ve said with a grain of salt, I’ve read a lot, but none of it particularly recent and like you mentioned, all of it would have been general or vet based or similar websites, not actual journal articles.