Implement {T190063} as approved:
When running links updates (after an edit, etc)
# for each slot, construct a dedicated ParserOutput, and also a ParserOutput for the combined output.
# merge the link tracking information from all slots's ParserOutput into a combined ParserOutput.
# run LinksUpdate with the combined output.
# run all other DataUpdates queued by Content objects with only their own Content/ParserOutput.
This means LinksUpdate will see all aggregated information like before, but newly introduced DataUpdates from non-main slots see only their own, unless they explicitly access or transclude other slots.
Rationale: This approach preserves the maximum of information, and is easy to implement. The fact that it may lead to extraneous data tracking and spurious purging of the parser cache does not seem relevant in the light of the currently targeted use cases. This issue should be revisited in the context of the creation of an entirely new mechanism for tracking dependencies of generated artifacts for purging.