Rebooting this closed topic:
I recently had a message from a TL3 user who told me he never clicked on the private categories because he thought the padlock meant he didn’t have access there. I was also confused the first time I saw that.
I really liked this icon, as there is just no way to misunderstand it:
I beg to diffeeeeeer 
Nothing is stopping you using CSS to change it on your own personal instance.
There are far too many interpretations of ‘man in jacket with raised collar, hat and glasses’, are they a spy? a private investigator? a rogue agent? what’s their intent? The name FontAwesome uses is in itself cryptic, how do secrets apply to something that ranges from sensitive, private or just non-public.
In comparison there’s only one for a lock. There may be better, but I don’t think that’s it. Objects are always good, they serve a particular purpose, people on the other hand are much more vague and open to interpretation.
How about a good ol’ mask?
Nothing is stopping the Discourse developers to make default settings that are less confusing either.
I would hold off on anything here till the font awesome 5 upgrade is merged … there are more possible icons to choose from. I don’t see us changing core though any time soon.
From my perspective that’s good news, there are already significant numbers of deployments and huge numbers of users who have become acquainted with the current convention. As I said above there may be better options, but it’s almost too late in the day to bother with an incremental gain which has any potential to cause more fallout than benefit.
There are people who never seen nor used floppy disks yet click that icon every day to save their data. Most computers don’t include optical drives as standard now, let alone 3.5", but it’s still very much the norm. I use certain tablet apps which bear the trusty floppy disk icon, despite the two technologies never having intersected.
If the perfect icon which requires zero explanation appears that’s a different matter entirely, but let’s worry about perfect tomorrow, for now good enough is good enough.
I had one instance where we changed the icon to https://fontawesome.com/icons/unlock-alt?style=solid which was the default in the home page.
That’s a great example of tweak which eradicates any suggestion that the content is locked away from the user, without introducing additional uncertainty.
Apparently this is still a problem for at least some percentage of non-old-timers. The Discourse defaults still have this conflict in iconography:
Closed Topic: You don’t have access and can’t post here. Available Category: You do have (special) access and can post here.… and I’ve had a few reports recently about people confused by the two separate meanings for the same glyph.
Is there any easy way to bulk-change these icons (for revealed categories) to the unlocked icon?
This Theme component does that nicely:
It seems much better. Why not change this in core?
Closed to focus discussion in Alternatives to padlock icons on secured categories