Opera recently sidebar extensions back in for the first time since the re-architecturing of a few years ago. That was a month ago and there are already quite a few extensions available: https://addons.opera.com/en/extensions/?tag=sidebar&order=po....

Vivaldi, basically a modernized Opera 11/12, has supported some sidebar elements from the beginning.

Since Opera and Vivaldi are both Chromium-based, it's not surprising to see sidebars getting into upstream.