Well, sort of, we're not deleting it anymore. But it can still get overwritten at any time (that's up to the updater to decide).
This is a part of the many "optimizations" made in the 4.8 version of the updater (especially for the automatic/background updates). We try to do only the bare minimum, to minimize the CPU cycles (and disk reads/writes) needed to perform the background update.
Cheers
Vlk