As a Roblox developer, it is currently impossible to let a player report another player through a custom leaderboard. This is because there is currently no publicly accessible report API.
If this was added, I would be able to bring my custom leaderboard closer to feature parity with the built-in leaderboard.
This can still be implemented safely in the same way that purchase APIs and other social APIs are implemented - just have Roblox prompt the player before actually executing.
I will say as cool as it would be for custom leaderboards. The main issue I see with it though is what stops people from just coding it so it targets one individual? For the most part I don’t see any real way to support official reporting besides just opening the report menu with nothing filled out, but as of version 643 roblox added a function to the Players service called ReportChatAbuse so maybe we may see some sort of official implementation soon.
These three features all have a common theme (enabling custom leaderboard functionality) but they are all distinctly different features, and more importantly, they may be in areas that are managed by different teams. Having separate threads means they can be triaged independently and handled differently.