From Brian:

The main issue here is that the protocol commands for uploading source maps currently require a specific recording --- we don't have a way to upload source maps and associate them with recordings after the fact. That wouldn't be hard to add, and the reason I didn't do so earlier is that without user accounts the only real implementation option is to associate the source map with any recording that includes the map's generated sources. That isn't exactly desirable --- people making recordings of a public website could get access to the original sources if the source map had been uploaded by someone else.

Now that we have user accounts, we should be able to upload source maps but lock them so that they can only be accessed by certain users. Then, when one of those users visits a recording we see if we have any source maps which that user is allowed to access and which applies to a generated source in the recording.

Here are some docs from Sentry