This is Mac only, right?

That's because they use (bad) compression in their backend, not because of the file format itself. Even if you select the highest definition settings when exporting your videos, they will always have a degraded quality after you've uploaded them to their platforms (especially Twitter—it transcodes the videos for delivery over the Internet and then adapts what quality you see at playback time based on your network's speed. On the other side, Instagram is maxed out at 30fps). YouTube or Vimeo, for instance, don't have this problem since they don't auto-play the clip when you scroll through their page. With MP4 you also gain playback control.

It doesn't need to be autoplaying. GIFs right now are not autoplaying. They play only on mouse hover over. Furthermore, WebM was made by Google especially for the web. But MP4 has a better support. And 8MB in 2018 is ridiculous for a 60fps GIF at an 800✕600px resolution.

Suggestion.

Here are a couple of recent opinions that you might be interested to read.

I'm glad that you've finally introduced grids, but they also need to have a relative/adaptive function, not only an absolute one, based on pixels and artboard size. It should be able to define a percentage based column, have the ability to align and also a max grid width.

Long shadow is not dead.

Why do they keep changing it each season? I wonder if they've seen this.

That option is only for the sidebar theme, as the feature clearly states. Only the left tab with your teams will be affected by the change. It doesn't modify the actual layout where you read and write messages (the place where you spend most of your time), which makes it hard to use at night on a large screen.

Why does Slack think it has no competition? I'm still surprised people are using Slack when they come with very little new improvements for their software. I'm not even a gamer but Discord is immensely better than Slack ever was, with new features being baked every single week. People were asking for a dark theme for Slack for ages and they stubbornly refuse to implement it. No wonder they get so salty at the sight of competition.