Hi @Arek1,
If you'd like to try out the absolute latest version of Mattermost, we run a server on https://pre-release.mattermost.com that we use for all of our day-to-day communications and to talk with contributors. If you're interested in trying out our latest full release (3.5.1), that's available on https://gitlab.mattermost.com. Both are open for anyone to sign up and try out.