The omitting of timezones doesn't matter to a vast majority of the world, since most countries only have one time zone so I don't see a reason why that is relevant in most use cases.

ISO is a general standard, it's in the name and the RFC is created for the internet, that is also in the name/description of the RF.

Using 2025-164 can be handy, I actually use the day of the year to check what invoices from previous year are open since those are the invoices that are due 164 days or more.