Keep in mind, the goal of RSS is and always was to serve as a sort of
shim between the simplest, most static of websites (the ones that live an entirely-static life, which don't run code during the rendering lifecycle, and therefore which can't call out to tell anyone about their new content) and users (who want to receive a notification or message somehow anyway.)
RSS was largely supposed to be a hidden bit of infrastructure, allowing some intermediary (like an RSS-to-email-digest gateway, or a news aggregator service) to scrape the RSS feed and convert that into useful notifications/events/messages in some form. The idea was that you, as an author, would pick one of these services, set up an account with it, and point it at your RSS feed URL; and then you'd put a link to that service on your page, which your users could then subscribe to, in place of subscribing to your site itself. Just like a regular email-notification service, but with the webhook part "turned around" so that it's the service poking your server, rather than your server poking the service.
"RSS Readers" were never meant to happen; RSS was never meant to be used as a direct link between authors and users. If you think about it, such a direct M:N link is exactly the opposite of what an author with a simple static resource-constrained blog wants—their whole user-base would now constantly be polling their (or their ISP's) dinky little server! (But it's fine in practice, since we eventually got Cloudflare and PuSH.)
If you're a content author with a full-on web-app, you can call out (with webhooks, or messages in a message queue, or even API calls to a service like Sendgrid or Twilio), and so, for you, RSS is unnecessary. As we've seen higher-level PaaS-style personal web hosts like Netlify, Squarespace, and WordPress.com supplant the old-school Apache public_html/-style hosting, fewer and fewer authors have needed RSS, because they've had access to other methods to deliver notifications directly.
As well, microblogging (mostly in the form of posting links to your new blog posts on either a Twitter account, Facebook page, or subreddit dedicated to your blog) also take the place of notifications of new content on the blog, as well as providing the bonus feature of moving the "comments section" of your posts outside the blog itself, such that you no longer need to host or moderate it. Many people today combine a static-site-generator blog on S3/Github Pages/etc. (having no real "front page", only at most an "all posts" archive for web spiders) with an external, mostly-ephemeral social-network-microblog "index" of their blog.
RSS still exists for those authors who it serves, of course; but it's always been an author-side feature, and so, if you're seeing it less, it's because fewer authors need it.
(And, if this annoys you as a consumer-of-feeds, know that RSS is probably not the universal-notifications-infrastructure you want. Please don't write gateways that scrape pages and spit out RSS. If anything, emit emails. Mail readers are a strict superset of RSS readers, and usually do everything RSS readers do better than RSS readers do those things; but that's a topic for another post.)