500 KB + Tens of millions of lines of code of a browser + a bunch of backend code. Want a lean Slack alternative? Use IRC. Want a fancy Slack alternative? Use Matrix.
What about a Google-searchable alternative to any of these? I think this would be one of the main selling points of Linen.
See discussion 7 years ago on serverfault.com, and the links therein:
How to set up a IRC server that logs all messages? https://serverfault.com/q/190069/113898
How to Offer Searchable IRC logs? https://serverfault.com/q/36886/113898
The bot option should be valid for Matrix as well, although there I'm not sure the bot world is well-enough developed to be 100% sure that a bot exist for every common desire like saving channel logs.