You need to do this on slack unless you pay for it. Comparatively IRC is free and it's actually easier to make a logging IRC bot than it is to make one on slack.

The time it would take me to research a logging bot, set up a server, add monitoring to make sure it stays up would cost more of my time than purchasing Slack for several years for my small team. I don't see how you claim it can be easier than Slack since Slack by definition has everything stored already, you just have to pay for it.

Quassel (a bouncer/client combination) has that included.

A one-click setup powers my IRC clients with multiple devices and logs even when I’m not online, powers a fulltext search ( https://gluu.kuschku.de/dl/videos/2016-09-16_04-03-36.mp4 ), powers a fully public logger https://gluu.kuschku.de/logs/ and much more.

All this exists, is free, and just works.

And there’s many more such systems that provide this functionality. From IRCCloud to many web clients.

key point: you have to pay for it

re IRC it's not a hard problem to solve whatsoever