It was brought up in another thread that the mighty-1284p core was a couple years behind. I'm not claiming to know what I'm doing here, but I did clone the repo and took a look through the commits. There are only a handful of the core files that are changed. Most of the changes concern the pins files, boards.txt, bootloader, etc., as opposed to the code in the "cores" directory. And the changes to those core files that are modified are not extensive.
I'm not in a position to make any promises, and I will continue to think about it, but it seems that an update would not be a huge amount of work. My purpose for this post was just to give that assessment of the situation and see how that matches with others' perceptions.
The core files that have changes are:
cores/standard/WInterrupts.c cores/standard/wiring_private.h cores/standard/Arduino.h cores/standard/Print.h cores/standard/wiring_analog.c cores/standard/WString.h