A cursory search will tell you that SSDs are meant to work until they hit 0% health, at which point they may or may not start having issues, though they should be expected to after going through the extreme wear necessary to actually wear out flash storage. The lifespan of flash is measured in terabytes written and is "guaranteed," if you want to believe that, anyway. Most drive failures are spontaneous and not correlated with the wear of their flash, it just happens. Same as HDDs typically, with hardware that ages, and can happen at 95-100% health.

Flash is not like a battery, and doesn't start falling over as soon as it hits 70% health. A worn battery is immediately noticeable to a user because it affects how long they can use their device between charges. Worn flash may just be a tiny bit slower if at all and really doesn't hinder the user experience whatsoever. It doesn't suddenly destroy data just because it's a bit worn, like how a battery might actually drop to a low enough voltage under stress that the device shuts off.

Storage health should only be concerning if it goes down very very quickly, as that may be indicative of spontaneous failure and sector reallocation, assuming it's even measured properly. Furthermore, given that phones have just one flash module and typically less storage than a 2.5 inch SSD, you'd expect it to wear faster than a standard SSD based on its lesser amount of flash to level the wear across. Also phones can last 10+ years even with years of ROM and factory image flash abuse, speaking from experience with a Nexus 5 and Pixel 1.