The only way that would happen is a bug. YC always responds to applicants. Rejections usually aren't personalized because that can't scale - but not hearing back at all should never happen.
If you want me to look into what might have happened there, I'd be willing to try if you email hn@ycombinator.com.
I double checked my emails.
Anyway, I'm realistic in knowing my ideas aren't going to garner investment.
I would love if you had a YC Junior for just getting help on ideas vs capital.
Your best bet is to go to onto Reddit, Discord etc, find the startup communities and ask for advice there. There are a lot of people who want to support other founders. Or even better go on Linkedin, find some prospective customers and ask them for advice. I've had about 10% of people reply back some with page long answers.
If you are willing to spend money there are plenty of services like Kintell, Intro.co which will allow you to book an hour with investors or successful founders. But there are plenty of free options that I would start with first.
That depends on what you mean by "specific."
Several Sand Hill Angels members, who are investors, provide feedback almost every month. (It's in the context of a pitch, but...)
See https://www.sandhillangels.com/raw
("pitch practice" is somewhat misleading.)
[edited to add some detail]
The typical angel-group pitch has three parts, the pitch, the Q&A, and then the "with the entrepreneur out of the room" discussion.
This event lets participants listen to that discussion, as well as those three parts for the other folks pitching the same night.
Then there's an "ask investors anything" session at the end.
In addition to the application, it's worth exploring https://www.startupschool.org/ if you're looking for a "YC Junior"