How to contact Apple to check if they will permit an iOS app?
The short answer is once you are a paid developer, you have paid support from developer relations. I can't imagine apple would offer anyone an unofficial or cursory "you're good" without seeing the app

The good news is Apple now has clearly worded and very specific public guidance on what app review considers and specific items relating to safety.


https://developer.apple.com/app-store/review/guidelines/


Apple’s review guidelines should help you decide if you want to pay the $100 to get a proper response from Apple on your specific implementation of a front end to that site. (Even if there is an API to let you host it - seems the site is having problems getting hosted and has moved to paid only access)

Section 1 on safety has all you should need to decide if “free speech” means you cannot or choose not to control bullying and hateful speech.


  1.1.1 Defamatory, discriminatory, or mean-spirited content, including references or commentary about religion, race, sexual orientation, gender, national/ethnic origin, or other targeted groups, particularly if the app is likely to humiliate, intimidate, or place a targeted individual or group in harm’s way. Professional political satirists and humorists are generally exempt from this requirement.


The bar for publishing free speech apps is that you would be held accountable for Apple’s definition of ensuring some level of safety for people that download your app.


  To prevent abuse, apps with user-generated content or social
  networking services must include:  
  
  
  A method for filtering objectionable material from being posted to the app
  A mechanism to report offensive content and timely responses to concerns
  The ability to block abusive users from the service
  Published contact information so users can easily reach you
  


Now, how you value your time - I would presume you'd spend 10x on legal advice so you understand the contract you sign to become a developer over the $100 Apple charges to be a developer.

The hierarchy of control is pretty easy to see, though if you step back from the details of app review. From most strict to least strict Apple services:


Most restricted is if your app contains items that might be adult or objectionable or hateful. Expect Apple to carefully look at an application that has  something widely understood to have racial undertones like a Nazi Swastica or a US Confederate flag. Based on the country in which you want to sell that app, expect to handle sensitive content with care.
Less restricted is if your app is a portal to user generated content. You absolutely have to disclose this, but there may be more latitude to pass review when you’re not embedding the content in your app.
Even less restricted is of your item is a book or music item - that is about expressing your artistic and intellectual ideas and not that Apple endorses your lyrics or writing.
The most free speech friendly expression is a web app - just make your Applications into a progressive web app or offline web app and there is no review. Anyone interested in participating in free speech is free to run your app without Apple or anyone else reviewing it. (Except perhaps the government where people live)


I would say, think about why your app has to be on the App Store and not a web app if it’s just a discussion forum or place for people to gather. Free speech generally restricts governments and not platforms and companies like Apple or even Stack Exchange which all publish guidelines restricting speech on their platforms.