Apple have recently published a couple of specifications documents for podcasting at:
https://help.apple.com/itc/podcastsbestpractices/
The Best Practices document discusses formats and loudness. Loudness is as expected at -16 LKFS, although they don’t mention anything different for mono vs stereo which I find a little odd. It is pretty standard practice for mono content to be delivered 3dB(/LU) down at -19 LKFS to allow for panning law loudness changes as per:
Why and How Your Podcast Needs Loudness Normalization – TAP307
It also recommends using AAC over MP3. I have traditionally stuck with MP3 as it is so widely accepted for playback across devices and platforms, but perhaps AAC is so widely accepted these days that that might be a better choice given it provides a bit better audio quality at similar bit rates.
The Marketing Best Practices document is something I’ll take a closer look at to see if there are some useful resources there.