Right now there's a potentially confusing/messy way to do partial (per-wiki) deployment of streams. "Potentially" because we don't have enough anecdotal evidence to suggest one way or another, but over time as the Event Platform is used more for instrumentation and developers see what the pain points in their workflow and instruments' lifecycle are, we'll be able to see if partial deployment of streams is an area for improvement.
The best approach relies on configuring the stream in default/metawiki, but then registering it with EventLogging (via `$wgEventLoggingStreamNames`) for the specific wikis you want it enabled on. (See https://wikitech.wikimedia.org/wiki/Event_Platform/Instrumentation_How_To#Per-wiki_configurations for more details.) Alternatively it's possible to deploy a stream everywhere with `sampling.rate: 0.0` and set `sampling.rate: 1.0` on the specific wikis you want it enabled on, but that's bad for performance reasons.
If it turns out that the EventLogging registration-approach is too confusing in practice, we could add a universally recognized setting (that is, EventLogging's `submit()` and Event Platform Clients for iOS/Android have logic to handle it) to disable a stream. Something like `enabled: false` or `is_active: true`