Note that that slide deck was put together by an individual whose pet cause is to constrain amateur radio to open-source digital modes only. That being said...I don't know that the Winlink team itself was promoting "security through obscurity", but there was open discussion on the pros/cons of Winlink where one of the talking points was "we can't encrypt on amateur frequencies, but the compression algorithm makes it difficult for casual listeners to read your mail." As noted elsewhere in the thread, the changes to create an online archive of traffic were an obvious response to complaints raised to the FCC about the difficulty in self-policing Winlink on the amateur bands. I would agree, however, that Winlink Express ought to caution users about the online archive. Folks shouldn't be sending "private" (as in "I don't want bystanders to read") messages via amateur radio to begin with, but disclosures of where/how your data might be used should be standard operating practice.