Don't know who currently maintains the standard, but the D stands for Deutsche - making it (translated) German Industry Norm.
I suppose it would make sense for some dimensions to be specified - so the tallest part fits through the cover aperture, and the lower parts don't foul the back of the cover - but terminal positions and characteristics would be a different matter.
I strongly suspect the gross dimensions are specified - after all, I was able to order a timer relay from one manufacturer, a box from another, and a DIN rail mounted blank from another, and they all fitted perfectly together. But since the timer relay was wired with tri-rated flexibles, terminal positions would be irrelevant.
Switch to CUs/DBs like we're discussing here and it's a different matter. I think components for these would come under a different standard (referencing the DIN standard in part). But there's things like, what happens if the terminal positions are standardised, but adjacent devices have different terminal clamps onto the solid bus bar ? Say one had a rising cage clamp, while the next has a fixed box and screw clamp.
Not insurmountable, but it would require a demand from industry (big outfits, not the individuals like us) to fund the standards making process and come up with a standard. The industry would then need to adapt products to meet the new standard - which might mean a new range of devices incompatible with existing installed products, which would impose a huge cost on the manufacturer and distributors.
Given that there's no advantage to any single manufacturer or to all of them collectively, why would any of them be interested in this ?
When I say there's no advantage to any manufacturer, I really mean that I can't see one. Why would (eg) MK be interested in installers/users being able to fit a (eg) Wylex device in an MK board ? Head down that route, and you quickly get to the question of - if the devices are now guaranteed to be physically compatible*, why can't we mix and match devices ? So another can of worms to pour out on the table. Address that, and you end up with manufacturers struggling to differentiate their products - all the same mechanically, designed to be (eg) magnetically tolerant of "foreign" devices either side, etc - and so it ends up being nothing but a race to the bottom of low prices, low margins, and falling standards. Oops, didn't I just describe the current situation
* One of the reasons for currently mandating only using devices from the one set of approved components is because of the risks of (eg) connections not being made properly if the busbar entries don't match between devices.