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.