You gotta use Sponge for now-a-days versions of minecraft, here is the link: https://www.spongepowered.org/
For older versions you can use cauldron here is the link: https://mcadmin.net/cauldron
KCauldron is the successor to cauldron and can be found here: https://prok.pw/index.html
FYI cauldron is discontinued but you can still download it.
Sponge is the new deal for modded minecraft server and has plug-ins and some plug-ins that will let you uses some of the plug ins from other api's
If your talking about adding plugins to a straight forge server other than above the closest things to it is, Forge Essentials and FTBLib, FTB Tweaks and FTBUtilitys "FTB = Feed The Beast" there are some other mods that add some things that some plug ins do, like BetterSleeping, MadHelp, netherportalfix, and PerfectSpawn, just google them, they all are on here, or on curse. But really there are not many "plug in like mods" out there and I say "plug in like mods" meaning what they do, not that they are like plug ins in the fact they only need to be server side each mod is different and some need to be both client and server, some don't, you will have to look into each one. Hope this helps.