But yes, phone screenshot is another strategy with much less grey area. I'm just becoming more and more paranoid of some potential defense trying to accuse my photo of being doctored, especially with more and more AI tools available.
(We are use zulip for chat which is better than everything else I have used since irc. But the search is too limited for someone who knows regexes.)
You can have an offline copy of emails and you can BCC them to your personal account if you want.
If you are willing to violate that rule or the message affects your work contract which you are of course allowed to archive at least in zulip chat that's very simple (for a software person). They have a straightforward REST API. IIRC you can even choose between markdown source and HTML rendered output.
Many reasons. First, chat doesn't exist. What exists is scores of incompatible chat apps.
I use WhatsApp but I consider WhatsApp messages throwaway because I keep losing them anyway. They are scattered across multiple phones with no way to merge them. Backups are platform specific. Exports don't contain any metadata and can't be imported.
"Chat" is a useless mess, not a paper trail.
For email, I have consistent backups with metadata across many email providers and email clients going back to 2008.
Chat messages tend to exist in one place only (vendors' servers), with maybe a transient local copy that gets wiped over time, or "for privacy reasons" (like Messenger switching to E2EE, effectively wiping cached history on any device that went through the transition). Chat message is an object, it's designed to exist in a single place, and everything else is a pointer to it, or a transient cache.
E-mails, in contrast, are always copied in full. You send an e-mail to me, you retain an independent copy, I get an independent copy, and a bunch of servers in between us keep an independent copy too, even if briefly. I forward your e-mail somewhere, more people and servers get their copies. I reply back to you, more independent copies, that also quote the previous messages, embedding even more copies that are even more independent. This makes it very similar to paper correspondence (particularly when photocopy machines are involved), i.e. impossible for a single party to unilaterally eradicate in practice.
And then chat vendors implement silly features like ability to retroactively unsend a message, force-deleting it from recipients' devices too (it may still exist in backups, but vendors refuse to let you access those, even with a GDPR request). In e-mail land, that's fundamentally not possible.
(Microsoft tried to bolt it onto their corporate e-mail software, but it only works in Outlook/Exchange land, and it's easy to disable (at least was, in OG Desktop Outlook - not the still broken New Outlook Desktop Web App). I discovered this when I once saw an e-mail I was reading suddenly disappear from my Outlook, which prompted me to find the right setting to disable honoring unsend requests.)
So, come discovery time, critical chat history may turn out impossible to find, and any deeper search will require forcing cooperation of the chat operator. E-mails, on the other hand, tend to turn up, because someone, somewhere, almost certainly has a copy.