In reality this looks far too basic to actually be a todo spec for a reasonable productivity app.

ie basic functionality I personally look for that I don’t think this would support would include:

* Projects with hierarchies

* Reminder dates

* Tags and contexts

* Multi-Step tasks

Hierarchcies are handled by 3.2.15 (https://datatracker.ietf.org/doc/html/rfc5545), and I suspect that could be leveraged to handle multi-step tasks; reminder dates are handled by VALARM in 3.6.6 (https://datatracker.ietf.org/doc/html/rfc5545#section-3.6.6). Tags could be stored in comment properties (https://datatracker.ietf.org/doc/html/rfc5545#section-3.8.1....), but there's probably a better way already in the spec, which also includes support for extension by adding non-standard properties (https://datatracker.ietf.org/doc/html/rfc5545#section-3.8.8....).

Fair enough - I stand corrected! It does look like that could probably handle those things.