Google has recently decided that soon everyone will need to migrate from Manifest v2 of Chrome Plug-ins to Manifest v3. The one big change for me other than some of the new syntax changes, was you can no longer inject scripts into webpages. A lot of the changes for Manifest v3 are around the security context for plugins, which is good to see. In the past I could append to a webpages <script> data, and then have the page process that script in the pages context, now all that processing has to take place within the plugin itself, instead of on the page. You can still add to pages, but it has to be more static content, instead of dynamic.
One change that creates for you is which browser context you are working in. If you are on the page, you can directly hit all aspects of the page, and do AJAX requests under the users context. Now, any scripting you want done has to be done in the plugin itself, and if you want to access a non-public asset, the plugin requires the user to login itself. If you attempt to inject scripts onto the page you will get a CORS error, stating its from a different context..
For the main plugin I dabble with and work on, the API I access is open. This allows me to not worry about the context I am working in the browser too much. If it was an authed API, I would have to worry about having the user auth to the plugin itself. I moved all the logic from a split context of the plugin doing some of the work, then handing high level data to scripts injected into the webpages; to doing all the work in the plugin, then injecting final results (HTML data), and assets I want to change onto the page. In the end, this leads to a cleaner solution, and centralizes all the logic.
A big added benefit I saw to switching from Manifest v2 to v3 was in the security review process that is done when you upload an updated plugin, you get approved faster than in the past. For me, I got my new plugin approved in around a day (note the plugin I was working on is relatively small).