Read the change log for 0.19.6 (0.19.7 is a very small bug fix release). The big change is the parallel sending of activities, though it's meaningless for us. It's for .world-> far away instances (e.g. lemmy.nz) as world sends more activities than network latency allows.

Other than that, some good bug fixes, like the controversy ranking changes that will be most of the downtime. The relaxed cookie permission in lemmy-ui is going to, personally, be a great relief. (If you've ever run into the issue of clicking a feddit.uk link but Firefox refusing to say you're logged in, this should fix that)

One issue though is that Lemmy now applies size limits to thumbnails, which is great for storage space, but some apps where build around the assumption that thumbnail_url in the API was just a local copy of the original image. There's also an issues with lemmy-ui that image posts without a thumbnail don't display as image posts, so I'm leaning very hard in the direction of disabling local thumbnail generation until apps update and the lemmy-ui issues is resolved.