@AzureProd no update as this feature request is “low priority” right now, mainly because:
Secret values will be available in the build (that’s why you set those). So if someone has access to the editor (admins do) they can easily print that by modifying the workflow and starting a build which either prints the value or just sends it to somewhere, or stores it as an artifact attached to the build … The point is that if you’re an admin, you definitely can get those values, even if the values are not presented anywhere on the UI.
IMO making it write-only on the UI might even be misleading; indicating that something is write only when in reality the value can very well be retrieved during the build / by an admin.
This is not a bitrise specific thing, this is true for every CI/CD service. If you can affect the build (e.g. modify a script which runs during the build) you can get the value of secret variables.
We’re not against making it write only on the UI, I just wanted to share why this #feature-request got a low priority mark for now.
Happy Building! 
P.S.: a related discussion: Allowing developers to modify workflows without exposing secrets