No one is eroding anything, please don't be melodramatic. Free speech is a protection from the government, not products offered by corporations; end of story.
You are free to not use reddit, or create your own reddit.
No, that's the first amendment. It does not equal freedom of speech, and the US constitution is not the be all and end all on how rights like that work.
There's plenty of material in the philosophical world about freedom of speech in regards to censorship by private parties and other people.
Well the concept of free speech I mentioned wasn't 'anyone can say anything anywhere with no consequences', since that's something practically no one wants for obvious reasons.
Obviously you should be allowed to boot people from your house for being an asshole. And many businesses should probably also have the same right.
But there's also the ambiguities that mean it should be as simple as 'private property overrides everything else'. Some companies are utilities, and can't dictate whether someone can use their service based on personal views (electricity companies, water companies, telephone networks, etc). Sometimes someone's role means they can't be that selective about what they hear, like how a judge found it was unconstitutional for Trump to block people on Twitter. And there are issues about either public vs private property (like for company towns) or instances where a company is so large and has such a huge share of the market (like a monopoly) that you could argue whether they should be able to pick and choose what they like.
Well, you can get as philosophical as you want. In the real world, for American companies and services, it comes down to basically doing what they want.
There is no real argument that a company like Reddit must adhere to free speech. No one forces you to use facebook, reddit, or any other website.
Freedom of speech is specified as preventing the government from enacting censorship (the act of stopping something before publication) in every constitution I have seen.
It does not mean "I can say what I want and you have to leave me do it".
There's plenty of material in the philosophical world about freedom of speech in regards to censorship by private parties and other people.