Nothing is stopping you from learning these topics on your own. In fact the vast majority of these topics I learned about on my own before encountering them in university.
The problem with asking mainframe questions is that they aren't as relevant in today's landscape. Meanwhile TLB is relevant for performance, UDP is a very common network protocol, consistent hashing is relevant for designing distributed systems, etc. Especially if you are just self teaching yourself you aren't going run into a chance to work on a mainframe unless maybe you use a mainframe emulator, but why?
The problem with asking mainframe questions is that they aren't as relevant in today's landscape. Meanwhile TLB is relevant for performance, UDP is a very common network protocol, consistent hashing is relevant for designing distributed systems, etc. Especially if you are just self teaching yourself you aren't going run into a chance to work on a mainframe unless maybe you use a mainframe emulator, but why?