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.
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.