The spec is nice. I love the recognition that dates may be imprecise (e.g., 2022-05 -- although I would count that as the last day of the period, 23:59:59 on 2022-05-31, rather than the first day of the next period, 2022-06-01). I like the thought that went into it and the nice presentation.
On the other hand, I do not think this is necessary or even all that helpful in the real world. You don't need a standardized format if you are writing to do items manually in a text editor. You just write them. In the vast majority of cases no one is going to see them but you. Time spent conforming to a spec is time wasted.
A formal spec also doesn't help much for apps to use this format. There is not a huge need for portability between to do apps, and for most users to do items are ephemeral and not something that needs to be archived and revisited. Even if an app adopted this, being tied to a plain text format would likely hinder feature development, including the ability to sync across multiple devices. Beyond that, different to do apps use different systems of categorization and tagging, so really in practice I doubt this will become any kind of standardized format.
So, all told, it is a beautiful effort, nicely presented, but I think it is unnecessary.