That's why I use Discord, the free tier has _unlimited_ history. The client itself is lightweight, and it's an incremental improvement on IRC in many other areas, as you would expect. Still using IRC of course :)
Discord is an Electron app, so while it's not as bad as Slack, it's still hardly what I'd call lightweight, especially in comparison to IRC/XMPP clients.
There's a lot to like in Discord, and it rapidly became the go to client for all my gaming related friends and activities, but it suffers from the same root pitfalls that Slack always has. It will inevitably run out of VC money (or whatever they're spending now) at some point and start wanting to actually turn a profit.
I imagine this problem would be remedied by an acquisition.. or at least masked... Twitch (Amazon) seems a likely candidate, considering it's usage by Twitch users, and also the fact that Amazon doesn't really have skin in the game currently in the chat market (do they?), and could use a product like Discord in their ecosystem.
Some time ago, Twitch bought Curse, probably for the Curse App that already does voice and text chat. Also viewing streams and mod management for a handful of games.
