OSDev.org

Is it fair to assume that, on PC BIOS based machines with CPU supporting long mode, INT 0x15, AX=0xE820 is supported?

Yes and no.

Yes, PC supporting long mode with BIOS should support E820 and it's fair to hang and blame the user if such function is absent.
However, No, PC supporting long mode may not have BIOS at all, it may boot with EFI (hopefully with BIOS emulation in these years, but not always, e.g. mac do not provide BIOS).

PS. yes I know mac is not called PC but the hardware is comparable.

In original post I mentioned (U)EFI, but decided to remove that part, as it didn't bring anything to the topic - "on PC BIOS based machines".