On July 18, 2022, Slack announced that starting September 1, search history for organizations on Slack’s Free plan will be limited to just the past 90 days of message history. Instead of a 10,000-message limit and 5 GB of storage, we are giving full access to the past
It is true that Slack like chat work-spaces are not a strong point of XMPP (it shines as a personal messenger), but you can set up a system that fulfills nearly all your requirements (I think search is only per room), with ConverseJS Fullscreen (or Movim), a Prosody server and the newish webhook component. Or with Gajim and Conversations as separate clients.
P.s.: “smoothly on mobile” & element ??? The Mattermost mobile client is also not exactly great. In that area at least XMPP is clearly better.
It is true that Slack like chat work-spaces are not a strong point of XMPP (it shines as a personal messenger), but you can set up a system that fulfills nearly all your requirements (I think search is only per room), with ConverseJS Fullscreen (or Movim), a Prosody server and the newish webhook component. Or with Gajim and Conversations as separate clients.
P.s.: “smoothly on mobile” & element ??? The Mattermost mobile client is also not exactly great. In that area at least XMPP is clearly better.