Thank you for your responses. It really makes me think about the meaning of portability:
Are you moving your identity? (e.g. implementing something like instance-agnostic user PGP keys)
Your data? The posts and comments you've contributed, which would only make sense with the context of the entire thread.
How would the contents of entire communities be migrated? I presume that's where the valuable content is for potential buyers either to drive ad traffic or train models.