Cauldron is a server based off Spigot/Bukkit code but with Forge Mod Support, It can be used with modpacks if wanted. this is a serverside and not used as a plugin or mod.
Sponge is also kind of the same but it's a mod, But there's also vanilla support, Would be nice to add these platforms under Plugins or with a checkbox marked for Lucky Block mods and also Modpacks.
Cauldron has versions from 1.6.4 to 1.7.10
While Sponge is 1.8 and onward. (I don't know if they already backported already but I am pretty sure they'll work on 1.8 and higher only.)
These can be used for moderation tools and more commands that could be useful likewise with Bukkit/Spigot Plugins.
It would be likewise for the user to know if there's support for that modpack or this modpack.
However, I can understand if you don't add Sponge for specific reasons, So it doesn't get mixed up with Bukkit/Spigot Plugins. But it's actually kind of best for people who wanted plugins and mods but they have to choose between either but with Sponge, You don't even have to.
The Sponge Project Website: https://www.spongepowered.org/ (Might as well also apply for your sponsorship if you want to.)
Cauldron DLs were hard to find but I actually found backups from Yive's Mirror. https://tcpr.ca/downloads/cauldron
Cauldron started working on a different platform called Sponge if you like to know that small fact.
Before you go saying "DIDN"T YOU MAKE THIS REQUEST IN MODPACK/PLUGINS/MOD SUBFORUM" The reason is I just realized that custom platforms don't belong in the section.
And also, If you are gonna ask "Uh, They already have this function, Just go to Plugins and pick Bukkit or Spigot" And my answer to that is FORGE MODS AND MODPACKS AREN"T COMPATIBLE WITH IT, And that's why Cauldron and Sponge exists to make that work out.
Before you say that it's a plugin: It's not. Cauldron is not a plugin while it's a server jar, While Sponge is a mod to make plugins based off it's SpongeAPI and it's for Forge and Vanilla.