Well, welcome to September 2022, where the answer appears to be: now. I just rescued an E-waste 2008-2009 MacBook Pro, max supported OS = 10.11.6, and I embarked on the search for a newer browser than Safari 11... and I came up to wall after wall. No support, anywhere -- everything is requiring 10.12 now.
Opera is actually the worst: Edge requires 10.12, Firefox requires 10.12, Opera requires 10.13!
I would have added a screenshot of the install error here, but it's not allowed. Neat.
"You can't use this version of the application 'Opera Installer' with this version of OS X."
"You have OS X 10.11.6. This application requires OS X 10.13 or later."