Security implications too. The storage device cannot be trusted to securely delete data.
To zap a drive you ask it to securely drop the self-encryption key. The data will still be there, but without the key it is indistinguishable from random noise.
On NVMe/PC type applications with a controller driving the flash chips… I have absolutely no idea. I'm curious too, if anyone knows :)
When it comes to what non bare flash drives do, you can start here: http://www.vldb.org/pvldb/vol13/p519-kakaraparthy.pdf
This paper is imperfect and the following citations are worth skimming. There's a cohort of similar papers chasing the same basic question in recent years that aren't densely cited amongst each other.
Go here next: https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.46... but note that's just a jumping off point to the more recent papers.
It's hard to gain a full understanding of this layer because it's the basis of intense competition, hence held closely by controller manufacturers.
I'm far from world expert on this, but have read a lot about it and can answer with what I know to the best of my ability.
I didn't downvote your sibling response, but I did ignore it since it provided neither any sources nor any context for why I should trust your knowledge. Apparently others were less kind on your short statement.
With the additional information in this post, I'm much more willing to accept it into my head — thanks for answering this!
So, the drives do indeed never "overwrite" data - they mark the block as unused (either when the OS uses TRIM, or when it writes new data [for which it picks an empty block elsewhere]), and put it in a queue to be erased whenever there's time (and energy and heat budget) to do so.
Understanding this is also quite important because it can have performance implications, particularly on consumer/low-end devices. Those don't have a whole lot of spare space to work with, so if the entire device is "in use", write performance can take a serious hit when it becomes limited by erase speed.
[Add.: reference for block sizes: https://www.micron.com/support/~/media/74C3F8B1250D4935898DB... - note the PDF creation date on that is 2002(!) and it compares 16kB against 128kB size.]