.inc file for PIC 18F4585 to compile Assembly in MPLAB X

MPLAB X 6.20 version installed 5 days ago.

I am translating code written in MPASM Assembly to compile with the new pic-as.

Looking for the respective .inc file, I carefully browsed every single directory in the XC8 tree of MPLAB X with no luck.

Where should I go to download it?

You can just copy an old one and stick into your project directory. The inc files are normally located in your XC8 directory tree in the include directory As normal just include pic18.inc and it'll find the right one foryou.