It is because Forge has chaged how their servers are made available. MineOS and the profile list generator tries to download forge from this list: https://files.minecraftforge.net/net/minecraftforge/forge/promotions.json
That page gives a 404 Not Found, which again means that MineOS can’t find any files to make profiles from, and drops forge.
The files are still available here: Downloads for Minecraft Forge for Minecraft 1.16.5