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.
Nostr, another federated social media protocol, kind of like ActivityPub (which we’re using right now), but different.