Jeffs0418 :
For a manufacturer to provide that info(attribute) with any certainty would extend R&D time unnecessarily.
Why do you think that? In order to estimate endurance, SSD vendors need a pretty good model of the flash memory they're using.
I did say "with any certainty". Endurance estimates are reasonably accurate longevity indicators based. Total TBW(terabytes written) is the number people look for here. TheTech Report actually did put this to the test on a number of popular SATA III units over 18 months until all failed.
https://techreport.com/review/24841/introducing-the-ssd-endurance-experiment/5 But that extended project did nothing to address what we are discussing here.
Moreover, power-off data retention is a design parameter of the flash memory circuitry, which the chip maker needs to test in some fashion probably before they even ramp up volume production. Remember that NAND flash is a charge storage device, so all you need to do is look at the self-discharge rate. It's not like you have to wait however many months or years it takes for the data to become unreadable.
Therefore it falls on you to look up the detailed data on the exact NAND flash used in whatever unit you are curious about and extrapolate from there.
That stuff is far beyond my area of expertise.
Jeffs0418 :
Probably because an overwhelming majority of snappy storage(SSD's) users actually don't use them for unpowered long-term storage.
I doubt that. I've had laptops that've sat unused for months at a time. I'm sure there are enough people that leave devices powered off for weeks or months that it matters.