Is it possible to create a list based navigation in SharePoint Online?
In 2013 we could use code to create it, is it a possibility here?
Everything seems much more limited.
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You can edit the left navigation for your lists. Just like this doc said.
And in Team site, a newly created list will have a navigation link created by default on the left navigation. Are you handling a communication site? Would you like to say a bit more about your current site and your goal?
In SharePoint online modern experience, you can use SPFx extensions to create custom navigation - Specifically Application Customizer.
You can find more information & sample codes to get started with at:
I will recommend you to check SharePoint out of the box navigation options as well. Using some of the options you can easily create mega menu or cascading menu navigation.