If

• Indicator is enabled
• Statistic "Time Since Last Rest" is at least 72000 ticks
• Time is night or weather is thunderstorm
• The player is above sea level (y=64)
• The player has no light-blocking blocks above head ("sky access")
• The local difficulty is greater than a randomly chosen value between 0.0 and 3.0 (client cannot know that value)
• Difficulty is not peaceful
• Gamemode is not spectator

then OneBar should show some kind of emoji while conditions are met. The option should have a hotkey for easy toggling because OneBar does not know if the world actually allows phantoms (mention that in the option too)

https://minecraft.fandom.com/wiki/Phantom

Proof-of-concept now available in the branch phantom. Essentially it works and even tells the chance % of it spawning but there are a lot of caveats:

• "Time Since Last Rest" is a serverside statistic that the client does not know in real time. That means OneBar would have to constantly ping the server for it (similarly to how statistics screen works each time you open it). Would it be acceptable to ping it every time weather/time changes to appropriate value? Even then it might be too often and raise suspicion in some game conditions.
• OneBar does not know if the world actually allows phantoms. That includes the gamerule doInsomnia, the Nether, End, custom dimensions, ViaVersion, minigames etc. So the user would need to toggle it a lot, which is obviously annoying.
• "Sky access" detection not yet implemented.
• It is very hard to pick an emoji for it, so right now only the abbrevation is used. The abbrevation "ph" also potentially conflicts with "piglin head" (any gold armor) abbrevation "pH".

It was fun, but it is probably not worth implementing right now.