I've noticed digital artifacting on radio broadcasts, particularly in rebroadcasting programmes from earlier in the day.

I've experienced this with multiple NPR affiliate stations where it's particularly noticeable for their generally high audio quality.

My suspicion is that audio is recorded with different codecs or at different bitrates, though I've never been able to establish this.

I've also not noted the problem for a few years, so it may have been addressed.

ISDN was 64kbps pet channel though - and to my knowledge did not use a wideband voice codec by default (or at least didn't mandate all ISDN receivers support wideband audio).

Lots of people who had ISDN kit plugged their old POTS phones into their TAs anyway.

ISDN is actually used quite a bit in niche markets in the states. For studios, typically you'll see a couple B-channels bonded to give someone 128K MP3 or AAC. At a bare minimum, 64 kbps/16khz g.722 is used. I worked in a newsroom a few years ago where I frequently had to record a weather guy from a distant area who used this.

Used in STE units for secure communication as well:

https://en.wikipedia.org/wiki/Secure_Terminal_Equipment

I think it sounded good because it was clean 64K end-to-end. No trickery with transcoding a bunch of times along the way so someone could save a nanopenny.

Right. The bits you send are the bits that are received, clocked at a fixed rate. No compression or timing artifacts.