Background: Long ago at University I was a mainframe user and spent some time learning about the IBM and compatible systems the university had and a bit about their operating systems. Later (but stll al long time ago) I was a long-time minicomputer user and sysadmin and had some exposure to higher-end minicomputers that were getting close to mainframe capabilities.
I started by giving quick reactions to the first bunch of example questions, but I think that's not what you want. I left the reactions below, just in case they're useful.
Overall, a bunch of the questions seem like "knowledgeable-beginner-looking-for-more-understanding" questions - looking for an overview, some explanation of why things are the way they are.
i.e. not a good fit for StackExchange, the answers require almost a book.
As for why people who are interested in that stuff aren't participating - there just aren't enough enough. Compared to the number of Windows Server or Unix/Linux servers, there just aren't a lot of mainframe systems around, so far fewer mainframe users/admins to (possibly) become interested in ServerFault or SE. Also, since the ecosystem is smaller (lots of IBM stuff, a few other suppliers), I would think most of the expertise is "owned" by IBM and a few vendors. If you want to know about TPF internals, you have to talk to IBM, there are no outside experts...
Here are some quick judgements: I wouldn't necessarily (and couldn't necessarily) defend these judgements, they're just my initial reactions...
In z/OS assembler, what does it mean (and what do I have to do) when I’m told my program needs to be reentrant?
Looks like more of a programming question
What is a dataset, and how is it the same, or different, than a file or directory on a *ix (Linux, Unix) system?
This would be considered too basic on many SE sites, the equivalent of "what's an inode in Linux"
I just did an
apt install hercules. How do I install an operating system on it?
Seems like too basic and too broad - How do I install Linux? How do I install Windows...
When JES2 complains of a SPOOL shortage, what’s the difference between TGS shortage, JQE shortage and JOE shortage?
I’m having difficulty in a multi-step job Involving GDGs. I create a +1 data set in an early step but in a following step, I am getting “not found”.
On ServerFault, this would fall afoul of our "troubleshooting" close reason.
What DISP parameter will let my batch job build a new loadlib while I have it allocated in TSOLIB?
How can I find out what "RACF group" I'm in?
Don't know, maybe ok, maybe basic?
What are the serialization mechanisms available in z/OS?
Seems like too basic and too broad - asking a broad question about OS fundamentals doesn't work well on SE
Where can I get a free prebuilt operating system for an emulated IBM mainframe?
I want to access MVS data sets using REST APIs ... how can I do that?
Both of these seem like shopping questions
When would I use JCL and when would I use the Unix System Services (USS) shell to perform a task?
Maybe ok, but might be too opinion-based, like the difference between unix shells.
I need to allocate a dataset. What does allocate mean, and what are record formats, logical record length, and number of tracks?
Another broad question about basics
How do I access data sets from the USS shell?
How can I create a Certificate Signing Request to install new certificates for web sites in a Sysplex?
Can I use my current open source tool chain for z/OS development? (It currently has Git, Jenkins, Ansible as the primary capabilities)
What are the fundamental differences between JES3 and JES2 ?
Maybe too broad, maybe ok... maybe there are a few really significant differences.
Why is the FORCE command dangerous? Is it safer to use the similar functions provided in monitoring software etc?