as an addendum (since people still find this thread): by now the download URL for core plugins has been documented: Installing plugins - OpenSearch Documentation
it is:
https://artifacts.opensearch.org/releases/plugins/${plugin}/${version}/${plugin}-${version}.zip
so, for example:
https://artifacts.opensearch.org/releases/plugins/analysis-icu/2.19.1/analysis-icu-2.19.1.zip
sadly, directory listing is not enabled, so you cannot manually browse it. also, the core plugin zips are not published to maven (where you can find other plugin zips).