I'm going to sum up my view here: if we split up repos to include plugins that depend on log4j-core, then we first need to define and release a stable plugin API. If we can't guarantee BC for plugins, then there's no realistic way to both split out additional plugins and have them be officially endorsed plugins. If said plugins were completely outside Apache (e.g., personal GitHub pages), then it wouldn't be as strict.
Also, I find it hard to believe that Google really uses a monorepo anymore. Such an idea wouldn't work too well with software that isn't continuously released. I suppose that's an alternative: I'd be open to splitting up repos however desired if releases were continually published to avoid version number synchronisation efforts, but I'm not so sure how well that meshes with the Apache Way for releasing software.
