Forge does not support Plugins, only plugins. -> We can not change that.
Combining two softwares (like Craftbukkit + Forge) is technically not possible -> We can not change that.
Cauldron (A software which supports combining mods and plugins) is not updated anymore and abandoned.
Thermos (A software which supports combining mods and plugins) is not updated anymore and abandoned.
Sponge (more precisely "SpongeForge") (A software which supports combining mods and plugins) is a completely different software with a completely different API. You can not run plugins which work on spigot/bukkit on a sponge server and vice versa.
Due to this the popularity of Sponge is very low and many of the plugins you want to use are not available for sponge.