Regardless of the merits of Signal, much of the criticism voiced in that article is nonetheless valid. Signal and other chat platforms employing the protocol are run as data-silos by design, using strongly identifying codes (phone numbers) as the required identifier — i.e., it is nearly impossible to create a throw-away account, because getting an anonymous phone number is no longer a practical possibility in a lot of (Western!) countries.
Whether or not such metadata is available to adversaries (either through OWS or gathered elsewhere) depends on who your adversaries are and whether or not a corporate entity can be trusted to be completely transparent about what kind of data they retain.
As for [0]; even if no relevant metadata is collected today by whoever owns the servers, nothing stops the party providing the service from doing so tomorrow.
Whether or not such metadata is available to adversaries (either through OWS or gathered elsewhere) depends on who your adversaries are and whether or not a corporate entity can be trusted to be completely transparent about what kind of data they retain.
As for [0]; even if no relevant metadata is collected today by whoever owns the servers, nothing stops the party providing the service from doing so tomorrow.