@jw-sbat Always make sure the plugins you are using are working fine before deploying to production. Even if the plugin doesn't specify compatibility directly it could still work. That property specifies a minimum nodebb version.
This is due to older socket.io clients (running on version 2) attempting to connect to the new socket.io 3. These protocols are incompatible, and so the connection is rejected, and NodeBB was configured to try indefinitely.
This is ok. Over time, as your users close their browser windows (or refresh), they will automatically start using the latest version, and socket usage will return to normal levels.
If you are worried about the excessive traffic, you can configure a nginx condition to drop requests with the query string ?EIO=3. Please see the issue linked above for more information.
@baris I started working on a more SEO friendly theme/plugin. I hoped I could keep all the changes within a lavender derived theme, but I cannot. Some changes will be in the core. FOr example, server side rendered pagination. I am looking for some advice on how to coordinate making changes in a theme and core nodebb?