This is now possible with Custom Tiles. You can either:
- Create a Custom Tile using the URL option for each site you want
- Use the Refreshable URL Embed custom tile if you want to be able to embed sites without having to define them in advance and want to optionally add a periodic refresh.
That being said, I’m looking for feedback!
Should this feature request be closed out?
The new Custom Tile feature functionally accomplishes the original request, but I’m contemplating if something like the Refreshable URL Embed custom tile linked above should be a native feature rather than requiring the import of a custom tile - basically making it more discoverable and easier to use.
The challenge is that many sites block being embedded and it’s not something that can be detected in modern browsers – different browsers handle this differently with some showing an error within the iframe and some not being very apparent that there even is an issue. As such, I’m wondering if coming to the community and seeing the discussion could actually be a helpful stop in the user journey.
For example, here’s what Chrome on a Windows PC looks like with a site that disallows embedding: 
