Nothing would happen. The SSD does not know nor care about your partitions. If you write data to any location, the data will stay at this location. If the SSD decides to shuffle blocks internally, it will copy the data to the new block, so it stays available at the same logical position.
If you do this and then the key is gone at some point, it was either overwritten, or the area was explicitely trimmed. This can happen when you repartition your disk in any way or if your partitioning software tries to be smart and trims any partition you delete and any free space between partitions. For example LVM does this when you set
issue_discards = 1 in your
It's not something the SSD decides on its own, though.
You still should not hide your encryption key in such a way. If it's obvious from your Initramfs scripts that the key is hidden in such a way, even if the exact location is not known, it could be bruteforced.