RevivalSMP is a community worlds server where players can upload their old Minecraft worlds and then explore them on a modern server.
A lot of old saves will not load on 1.21.11 so this server uses an automated conversion pipeline. You upload a zip and after about a minute it outputs a playable 1.21.11 world.
They even got a Better Than Adventure world working which is a Beta 1.7.3 fork with weird block IDs and a custom chunk format. They built their own converter to turn that format into Anvil region files and then Minecraft handles the rest with forceUpgrade.
It is run as a Docker service with an API and a server plugin that picks up the converted world so players can jump in and explore.
