‘Transparency’ (and privacy, security too) is possible only if the the apps (if server is involved, server app too) is released under a libre license either under FSF approved or at least OSI approved licenses. We cannot certify something based on developer’s promises alone. “trust us because you can trust us” doesn’t work, doesn’t make sense and is a major reason why we have these problems. Libre licensing should be a requirement.
Proprietary softwares/services - another major reason why we ended up with the current problems we aim to solve. If we replace one proprietary app X with another proprietary service or app Y, now we have to TWO things to worry about and it will not really be an ethical HT replacement.