I edited my original comment because others have pointed out it doesn't run afoul of antidiscrimination laws. You acknowledge that this behaviour is morally wrong, but don't say whether or not a platform should have a responsibility to prevent this behaviour. I believe they should

While the mechanism by which systemic discrimination occurs is different because it's individual users instead of the business, the impact is the same as businesses/public spaces discriminating against individuals.

This is because common social spaces are barred off to people of certain ethnicities and that means they can't fully engage in civic life.

Here's another question. Let's say a mall is composed entirely of pop up stores that close at the end of every month. These stores invariably ban visible minorities from using their services. Attempts to sue the stores fail, because the store no longer exists by the time you gather the information necessary to sue it. While the mall itself does not promote racial discrimination, it is impossible for visible minorities to shop at the mall.

Should the mall have an obligation to prevent discrimination by its tenants?

I would say "yes". In your bar example, it is still possible for a Catholic to get a drink at the bar. In the mall example, it is impossible for a minority to shop.

On Reddit, if it is impossible for someone to be Jewish or Israeli because they are banned on sight from most subreddits, that should be illegal.