Actually, the Speculative Loading plugin is still useful to have installed. It reuses the infrastructure that is now in core, but it provides a UI and other enhancements on top of what is in core. By default core uses the conservative eagerness with prefetch mode, but the plugin allows you use the moderate eagerness with prerender mode. It also allows you to opt in to Speculative Loading for logged-in users.

For more, see the plugin readme.

In general we make sure to plugins are kept up to date with whatever gets merged into core, so either they will stop doing anything on their own, or their purpose will evolve to iterate on the feature that landed into core.