Is there an archive of previous Challanges?
Wich was the first and Wich was the last?
It could be useful for new agents interested in decoding to have this kind of resources
Decode Ingress has walkthroughs and examples of how various passcodes were decoded. The site is now defunct (and appears to be down, actually), but you can access it through Wayback Machine. It has a lot of the old puzzles from 2018 and earlier, basically. I don't know of any archives of the 'challenges', they likely exist but are scattered. Decode might still be helpful for understanding the general passcode format and common things used though (although the format changes and the ciphers are a toss up. Could be ROT13, could be triple DES, could be something stupidly obscure that chains across a half dozen vague hints)
Back when I still had the capacity to do so I'd contribute the odd article on https://ingress.codes/ which is still up and a great resource for standard and event puzzle solutions (shameless plug) but it gradually ground to a halt when, let's say, Niantic's focus started to shift away from decoding challenges.
If it's appropriate and there's demand for an Archetype writeup in March this might be the place to put it.