Same here on Linux - Opera 135.0.5973.92, Linux Mint 22.3. Sidebar panel zoom is locked at 100% and nothing reaches it: per-site zoom on the panel's site does nothing, changing the default page zoom (tried 90% and 75%) does nothing, and removing the panel and re-adding it does nothing. On reload the panel shows the right zoom for a split second, then snaps back to 100%.
I use WhatsApp in the sidebar all day and need it around 80%, same as the Slack case above. Could this at least be made optional - per-panel zoom, or a setting to let panels follow page zoom again? Locking it at 100% fixed one complaint by removing something other people depend on.