You say that it is sorted in the order of most significants, so for a date it is more significant if it happend 1024, 2024 or 9024?
Most significant to least significant digit has a strict mathematical definition, that you don’t seem to be following, and applies to all numbers, not just numerical representations of dates.
And most importantly, the YYYY-MM-DD format is extensible into hh:mm:as too, within the same schema, out to the level of precision appropriate for the context. I can identify a specific year when the month doesn’t matter, a specific month when the day doesn’t matter, a specific day when the hour doesn’t matter, and on down to minutes, seconds, and decimal portions of seconds to whatever precision I’d like.
To be honest, no. I mainly know about JPEG XL only because I’m acutely aware of the limitations of standard JPEG for both photography and high resolution scanned documents, where noise and real world messiness cause all sorts of problems. Something like QOI seems ideal for synthetic images, which I don’t work with a lot, and wouldn’t know the limitations of PNG as well.