I was referring to using rack-mount Mac Pros in datacenters obviously.. not regular chassis Mac Pros. Or any actual, real life server application - with rack mounted Macs. I'm NOT talking about using Mac Pros at datacenters, Jesus...

I don't understand how could you misinterpret my message like that. I dont think they have their own name, Macracks comes into mind
I've never seen anyone actually using these rack mount Mac Pros - hence the question from my original post - if you know any examples that they've actually been used somewhere, not just for show / as Apple's PR stunt. To me it's a dead product, I've never heard of anyone buying it, using it, etc. So I don't know why you brought it up

Remember that applications you've listed, like military, scientific ones etc won't EVER come even close to a Mac - for safety reasons. They run proprietary UNIX / LINUX builds. Especially military. Would you imagine USAF servers running on Mac OS? That would be hilarious.
"Slots aren't >>REALLY<< a feature" - because they are something soooo obvious, it can't be considered anything other than an obvious necessity. And no - adding slots to current Mac silicon won't cut it for pros.. That would be too many shortcomings for what, twice the price? Also that's the exact reason why we haven't seen a Mac Pro yet - because Apple knows this and agrees with this statement. And if Apple doesn't figure out how to deal with this situation, we won't see a Mac Pro yet. Not until they do something with GPU - as they need to step up their GPU game anyway, because that's where the tech market is heading - high amounts of parallel mathematical operations in AI / VR / AR.
Also my question stands - who exactly a "Studio Mac Pro with slots for double the price" would be a viable, good option?