Yes, i can add that but why? All servers basicaly use PlaceHolderAPI and player expansion.

the idea would be useful if a person wants to implement support for SternalBoard in a minigame plugin (an example is a bedwars or skywars plugin), the custom placeholder system would allow you to create for example a placeholder "%skywars_on%" with a refresh interval of 1000ms that returns to you the number of players inside a certain arena (like 4)

But this placeholders should be created by the plugin, the scoreboard just parses the placeholders.