We're under no legal requirement but I simply don't trust Slack employees to be good stewards of our data. On a UI/UX basis, Slack and Discord are far superior to Mattermost, so hopefully they can use this funding to close that gap.
YMMV, but for me that's a very low value feature.
https://www.mattermost.org/why-we-made-mattermost-an-open-so...