I have two steam accounts, and I was not able to see anything related to a game marked private from my second account except when family sharing was enabled between the accounts. With family sharing on I could see all private games from my primary account on my secondary (including games which were not installed on the local system).

If you have family sharing on, hold off. Otherwise as far as I know it works as intended.

I’m not sure to the extent in which they are private, but in my testing they DON’T appear in the following places:

• The “Steam Replay” thing.
• Whatever ProtonDB uses to query your owned games.
• Your recent activity (it also doesn’t also doesn’t count your playtime when displaying the “total time play time” in the last 2 weeks).

Not sure if they are hidden in your owned games list on your profile, but I assume they would be.

Note that the count of games you own (which is public) does seem to include hidden games, if that’s a concern.

Second steam accounts are free to create. You can’t go wrong that way

It would probably be better if you said the game that you wanted to do this with.

and

Private is subjective. Do you mean as solo play or with friends. Most games have options to cover both instances.

Going to be honest did not know this was a feature.