[omarhaneef]

Can’t it just scan the client? For email at least, I feel it can just read your inbox or whatever and store the results of the analysis locally.

You were very active with this friend for n years and it dropped off. Drop them a line.

[derefr]

That presumes the client stores anything locally + in plaintext. Clients like Facebook Messenger, Slack, and Discord have no local storage other than (opaque, fragmentary, useless) cache databases, because they're just "webapps in a can"; while many of the more modern native chat clients like WhatsApp, Signal, Telegram, etc keep all their synced state encrypted-at-rest on disk.

Also, that presumes you're on desktop. On mobile, you just can't poke into another app's sandbox/container that way (unless the target app hasn't explicitly granted made some part of its container-filesystem externally accessible; and there's no reason these apps would.) And many people only have a mobile device.

If you're okay with a partial solution, a far easier one would be scraping the data out of the few clients that have web-app versions through browser extensions that read the state out of the page they're running in.

[omarhaneef]

Yes, good point and I like your final partial solution. I think it’s a good compromise.