Mastodon and RCS are lightyears from consolidating X/whatsapp/messenger/telegram/signal/discord/slack/teams/etc.
Email+notifications is a joke, lacking groups features, true undo, large attachments and video codecs, etc.
Email is immensely concentrated amongst a literal handful of large players, and intercommunications between those are fraught. (My Protonmail messages to Mozilla Pocket's QA team are rejected by their Gmail service, and have been for years.) Spam, confidentiality, privacy, and trust of asserted identity are all overwhelming problems. I make all but no personal use of email and haven't for years, despite first using the medium in the 1980s.
RSS has been strongly deprecated by major players. Its one bright point are podcasts (based on RSS), though that is also increasingly centralised and commercialised. The number of pods which are ad-free has been ever-shrinking, and those ads are exceedingly poor. Major publishers managed to make their RSS feeds all but useless (teasers of stories, abysmal formatting, ads/nags). Small blogs are still a source, but they're hardly the vanguard they once were.
The Web's standards process has been utterly captured by Google on all three of the publisher, ad/tracking, and browser endpoints. Scrap it and start over is my general recommendation. Gemini (the protocol, not the AI engine) is an interesting but insufficient start. (I'd like to see more structured documents with notes/references at a minimum supported.)
So yes, open standards still "exist" for some value of existence. But they're far from healthy.
Email: One of the oldest parts of the Internet. Very open standard. Federated. Largely ad-free. Little lock-in (Though @gmail.com addresses are a potential serious risk). Lots of attempts (by Slack, etc.) to "kill" email because no corporation controls it.
RSS/podcasts: RSS (or Atom or whatever) should be way more popular, but it still lives on through podcasts where anyone can publish anywhere and subscribe to anything. hough Spotify and Apple are trying hard to lock things down, they haven't succeeded yet.
The web: Exists and is still largely open. Efforts to turn everything into a closed app haven't succeeded yet and attempts to lock down the web (e.g. web attestation) have failed so far.