⠀This guide lets you nickname any user on Steam without having them added, so you can keep track
⠀⠀⠀⠀⠀⠀of certain individuals in the community even if they change their username and avatar.
⠀

⠀⠀Open a profile in a browser and send this code in the console: javascript:ShowNicknameModal()

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀That's all you have to do for this method, credit to Mangalhito.
⠀⠀⠀⠀⠀⠀⠀⠀⠀ If you don't want to use a browser, an alternative method is provided below.

⠀⠀⠀⠀⠀⠀⠀⠀You can now easily view your given nickname to users you don't have added.

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀Navigate to their Steam profile and send them a friend request.

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀Open your Steam chat and click on the waving friend icon.

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀Scroll to the very bottom until you reach outgoing requests.

⠀⠀⠀⠀⠀⠀⠀⠀Right click the user you sent a request to in step one and select "Add Nickname".

⠀⠀⠀⠀ This will add a nickname to this user. You can now cancel your friend invitation if you want.

⠀⠀⠀⠀⠀⠀⠀⠀You can now easily view your given nickname to users you don't have added.