While the PSVitas lifespan has more or less ended, a few games and updates are still trickling through and these may require firmware 3.70 or above thus rendering them unplayable on earlier firmwares as reF00D only allows you to launch games requiring FW 3.69 or earlier on these FW versions. That has now changed as the developers behind it, which include scene veterans dots_tb/CelesteBlue/Princess Of Sleeping, have released an update to its keys.bin which modifies the artificial firmware range hard-coded into it since the keys on FW 3.70+ are still the same as FW 3.69s.
With the modification, the maximum firmware version has been changed to FW 3.99 which probably neednt be changed again as the PSVitas firmware updates are unlikely to reach past the 3.xx series. As a result, its safe to say that all games and game updates requiring firmware 3.70 or newer, such as heck-Diver, can load on older firmwares such as 3.60 and 3.65 unless Sony decides to change firmware keys which it hasnt done in the last few firmware updates!
To use the updated keys.bin file, all you have to do is install reF00D normally but use the just-released keys.bin file instead of the ones linked to in the installation instructions.
You can download the updated keys.bin file for reF00D from Team CBPS Forums by following this link.