extension-list | Simple GNOME Shell extension manager in the top panel | Theme library

Roaming Extension Manager is built in Visual Studio 2017:

The Roaming Extension Manager helps you keep track of all your favorite extensions across all of your development environments. Roaming your extensions keeps track of the extensions you have installed by creating a synchronized list in the cloud.

See also https://devblogs.microsoft.com/visualstudio/roaming-extension-manager-control-and-consistency-down-to-the-last-extension/

Given the useful info by @wOxxOm and @err1100 I've decided to self answer this question. I like using comments in the question for clarification, however I often see SO users answering the question in the comments instead of posting an answer (maybe because the answers in the comments are usually more conversational than stating an answer) but either way I think questions deserve answers. If either of them posts an answer I'll accept the first to post or if someone other than them posts a significantly better answer (don't go stealing their credit).

Persistent pages apparently have better support across different browsers and so should be preferred in that case (at least as of 11/24/2018). Otherwise consider using an event page if the extension is just for Chrome as persistence is rarely needed.

Context menus and anything with the declarativeXXXX naming scheme should be initialized once in the onInstalled event but there aren't many other things requiring one time initialization.

With persistent pages the script is loaded once and never unloaded unless the browser is closed and so onStartup doesn't have much of a use.

As said above, since the script is loaded only once per browser restart (and also ran once if installed on an already running browser) all per browsing session initalization can be done here.

I didn't have a clear idea of exactly what the use of non persistent pages were (I believe this is what wOxxOm is referring to as an event page as googling it takes me to defunct documentation and redirects to documentation without the term) prior to this question so I'll clear that up for those who may be in the same boat. A non persistent script is ran (at some point, IDK and won't be testing this) and registers it's event listeners. After that the script dies but the listeners remain meaning that initialization can be done with onInstalled and onStartup (since I know for sure when these are run and not when the non persistent page script is ran) and all the events you registered to listen for will reactivate that part of your script (really just runs the callback provided but whatever).

Just like with Persistent pages use this for the same things that require one time initialization.

I'd leave no code in the script and everything in a listener if creating an event page but I'm not well versed in event pages so I'll update this if someone comments on it being wrong. Stuff requiring initialization each browser restart would be in this listener.

As said above, I don't have a great understanding of event scripts but it seems like the only code in your script should be for setting up listeners and whatever variables those listeners need. No significant scripting should occur in the script however do as you must to make your extension of course (or just use a persistent page).

You can retrieve a param in query string as follow: