We love Mattermost for our organization; it feels nicer to use it over Slack/Discord because we're also FOSS. Our next iteration is to be moving to a more protocol oriented method - which means Matrix or ActivityPub even.

We don't care if our chat is search-indexed; it's mostly internal; there are client channels, specific topic channels. At least for our case being OUT OF SEARCH is better. However, we do use the conversation as seeds for our help-docs, blog post, etc, etc.

Perhaps Linen could be the winner by putting Slack+Discord+Mattermost+Matrix+ActivityPub in to a better search-index.

Anyone remember MARC? -- https://marc.info/