You already get this with either Redis or Keydb. The problem is that to avoid the disk being the bottleneck you must write to RAM, then later to the disk (you can't wait for confirmation that it was written) which means that even though a key may successfully be written to the DB, it hasn't been confirmed to persist.
Basically redis can't necessarily guarantee data safety - which is perfectly fine for it's normal use case as a caching layer and data you don't necessarily care if you lose (for example rate limiting relies on tracking the number of requests/s within say an hour - if you lose the data you don't really care). User sessions too, worst case people have to log in again, meh whatever.
The major bottleneck for most applications using Redis is the RAM capacity, not the write performance. Disk persistence can be more than fast enough as proven by dozens of key/value stores out there.
Basically redis can't necessarily guarantee data safety - which is perfectly fine for it's normal use case as a caching layer and data you don't necessarily care if you lose (for example rate limiting relies on tracking the number of requests/s within say an hour - if you lose the data you don't really care). User sessions too, worst case people have to log in again, meh whatever.