I think it's actually just a containers feature now: https://news.ycombinator.com/item?id=24850878
I use per-domain isolation, so I don't run into this issue.
However, I need 2 manual persistent containers for sites that are broken with per-domain isolation (Gmail and Outlook 365). Both visit many different domains after login. Within those containers, I need to be careful not to click on external links, but to open them in a new temporary container.
If I'm in FB Messenger and click an external link I don't want that link to be opened in the FB container.