It doesn't have the hyper-local capabilities that reddit has but metafilter.com deserves a shoutout. It has an established community, has been around for _ages_ and is a good replacement for /r/all and the like.
Check out an app called Artifact, if you're looking for that infinite scroll on published articles relevant to you.
It's made by the original founders of Instagram, and they're using AI in some interesting ways, e.g. rewriting clickbait headlines. Their Reader View is nice too, though I wish I could set it as default on each page open.
Unfortunately, lack of a website I can use on my desktop browser makes this unusable for me. Also I am not sure any of the user generated content on this app would be discoverable by search engines. I fear it will be locked in forever.
4chan? It doesn't touch the scale and breadth of Reddit and I was always a little uncomfortable the handful of times I visited. Maybe because of no-rules /b/ but there are definitely some wholesome boards out there.