There are a couple I have in mind. Like many techies, I am a huge fan of RSS for content distribution and XMPP for federated communication.
The really niche one I like is S-expressions as a data format and configuration in place of json, yaml, toml, etc.
I am a big fan of Plaintext formats, although I wish markdown had a few more features like tables.
I think CalDAV (which uses the iCalendar format) may be the closest thing. It covers calendar items, obviously, but also task and journal items.
Do you know if it allows dependent tasks?
Yes, but not all clients expose dependent tasks (which is sadly a common issue with open standards: they aren’t always properly implemented). I’m using Tasks.org on my phone (which supports dependent tasks), synchronizing to a Nextcloud server with the Tasks app (which supports dependent tasks now,
but didn’t for a long time), which also syncs to Thunderbird (which does not appear to show dependent tasks as dependents).Edit: remembered that the Nextcloud Tasks app has long supported dependent tasks. I was thinking of recurring tasks, which it does not support. Again, open standards aren’t always fully implemented.