This problem was most prominent right before the switch to 3D NAND, when planar NAND dimensions were at their smallest and the consumer market had already mostl

This problem was most prominent right before the switch to 3D NAND, when planar NAND dimensions were at their smallest and the consumer market had already mostly switched over to 3 bit per cell TLC rather than 2bpc MLC. In the worst case, we were down to about 8 electrons difference between cell voltage states. That's now been relaxed by 3D NAND allowing for larger cell sizes, and most 3D NAND also switched from floating-gate to charge-trap cell design so leakage is less of an issue. Nowadays, data retention in SSDs is only a concern toward the end of their lifespan (as measured by write endurance), and it's probably inadvisable for the SSD to start doing background data scrubbing until the raw read error rate starts climbing.