Reddit and HN fully support RSS.

Discord, WhatsApp, and iMessage are all messaging applications that aren't directly related to the use case for RSS.

That leaves Twitter and Instagram as the two major sites for which RSS would be applicable, but which don't natively offer RSS feeds. And a cursory web search reveals the large number of solutions people have come up with for subscribing to content from Twitter and Instagram via RSS, indicating that there's significant demand for it.

It's also worth noting that with Twitter in decline, the main competitors gaining traction, BlueSky and Mastodon, do both natively offer RSS feeds.

On top of that, the entire podcasting ecosystem is fundamentally based on RSS, and it's still the primary mechanism for syndicating blog content.

So RSS is not just alive and well, it's thriving.