My chat logs are just plain text under ~/.weechat/logs and I search them using rip-grep. Works really fast. I am guessing a nicer interface like IRCCloud or TheLounge can work like Slack for searching, file/snippet hosting etc in a more approachable manner.

So, you come to a new job and join a channel that has been around for 2 years. Design decisions and discussions from 4 months ago with the old tech lead are there. Does weechat give you access to those logs?

No, not by default but if your organization uses IRC then it would be near guaranteed that whoever set up the IRC server will enable logging on all channels and then likely give you a web front end to it so you can easily read and search through all of the history.

In other words, yes, in any reasonable environment you would have access to everything ever said in any logged channel.

So, someone is tasked with building and maintaining a small part of what Slack provides as a service?

I agree to what nickjj said but hopefully decisions like that are not taken in chat but rather on a mail or an issue so it is documented.