It looks like, in order to remove a data source, you have to delete it. To do that, you have to delete all its metrics. But if you still want to preserve the historical experiments, it seems like you'd be stuck? Is it possible to remove the data source without breaking those? Or could it be an option to archive a data source instead?

Yeah, archiving is definitely something we can add support for. One slightly hacky workaround is to make a new project called "Old Data" or something like that. We recently added the ability to restrict a data source to a specific project, so if you do that it won't show up as an option throughout the app unless you specifically select that project in the left nav.