Base components are still useful, just not in your UI library
alicepackarddesign.com
alicepackarddesign.com
The idea is you create a "base" component (let's say a button), then create more components that wrap the base component and apply overrides on top (like a different background color). If you want to adjust the size of all your buttons, you edit the base component and your changes propagate to all the other components. [1]
This of course leads to a tangled mess of dependencies when you start building complex UIs in Figma, but it is the only way to make mass changes without having plug-ins automate it, or doing the tedious task of manually updating a bunch of components one by one.
I used to use .base components before, but IMHO with the latest Figma updates they are not needed anymore. Editing variants is superfast with the layer multiselect tool.
I created and maintained a fairly large design system that I did entirely using .base and it brought more pains than gains, so I redid the design system using a more modern approach and with component properties and some js magic and superhappy :)