The paths have never changed - custom plugins and core plugins are different from each other and are located in different places. Core plugins are included in Grafana and are located in
public/apps/plugins if you have git cloned the repo or in
/usr/share/grafana/public/app/plugins/ if you installed Grafana on Ubuntu (see the installation pages for the file locations for other OS).
The docs say:
Clone an example plugin into /var/lib/grafana/plugins or data/plugins (relative to grafana git repo if you’re running development version from source dir)
Both those paths are still correct for custom plugins.