Where can I make requests to reopen questions on the Super User main Stack Exchange?
Right here on Meta. This is what the tag guidance says for reopen-request:
Requests to have closed questions reopened by community or a moderator. Please include arguments – why should the question be reopened?
I had such a request closed as off-topic and I don't understand why.
You don't give a request (or reasons) to reopen it. It's titled "Heavy Handed Moderation" and you spend the majority of the question body calling out a moderator's actions as "unwarranted pedantry". You added a sentence requesting it get migrated (not reopened, btw) as a secondary point to the rant - after it was already closed.
A proper request, for your specific circumstance, would look something like this:
Please consider migrating this question to Engineering SE
I have the following question about LED screen technology [link to deleted question], which was closed as off-topic. Despite clarifying edits it remained closed, and is now deleted.
I have determined that it's on topic at Engineering SE [link to Engineering meta], and I'd like it to be migrated there. Can a moderator please undelete & migrate it there instead?
This is not a guarantee that the request would be handled the way you want it to be - you'd have to be willing to accept 'no' as an answer.