Chrome, Edge and Safari since March 2023; Firefox since February 2024.

https://caniuse.com/mdn-html_elements_template_shadowrootmod...

Seems like browser support is pretty universal, even says so in the article

> All browsers support streaming HTML

And the caniuse is promising: https://caniuse.com/?search=slot

<slot> has been around for a while in Web Components; the relevant feature here is probably the (much newer) shadowrootmode attribute: https://caniuse.com/?search=shadowrootmode

But you can already do a lot with regular streaming and using CSS Grid, inline <script> tags, etc to move stuff around the page.