Highlights include Sliding Sync (instant login/launch/sync), Native OIDC (industry-standard authentication), Native Group VoIP (end-to-end encrypted large-scale voice & video conferencing) and Faster Joins (lazy-loading room state when your server joins a room).
You had me second-guessing for a minute, but I think the other commenter is correct.
One can definitely use Spaces in other clients, even Beeper supports them. So if it was an Element-specific feature, it doesn’t appear to be any longer.
Yeah, spaces need server side support too. Technically they are just rooms that are handled differently, but it’s not just a UI feature.