It aims to provide a definitive and less buggy experience, including backports of some modern conveniences (such as item smearing and ctrl sprint) and retromods that didn't exist back in the day.
Rewind packs are compatible with the major FTB pack from their same version. You can load an FTB Ultimate world in Rewind Upsilon and it will upgrade seamlessly. You just get a less buggy and more featureful experience.
So, why? It's pretty simple: these old versions of modded Minecraft are quite unique on their own, especially compared to modern modded, and the experiences offered by them still hold up today. Rewind packs are for anyone that wishes to revisit older versions of modded Minecraft, nostalgia or no. Playing a Rewind pack shields you from many known bugs, ranging from mild inconveniences, to game crashes, to dangerous remote code execution vulnerabilities. Additionally, all the mods are fixed to work on modern Java 8 and disable all reliance on long-gone servers. Upsilon is Rewind's flagship pack (and first pack), built for Minecraft 1.4.7. It is also the first ever modpack to use NilLoader.
Rewind Upsilon will be released on Modrinth once some redistribution issues are sorted out. You can try the beta right now in Prism Launcher by importing this instance zip — that version uses unsup to download mods from their original sources. You can also download this zip to run a server, with full updater support.
This webpage falls out of date pretty quickly — Rewind Upsilon is an ongoing and fairly large undertaking. For all the latest info, please check out the git repository which includes a full list of mods and changes, stop by Una's discord or its bridged Matrix space, or participate in our forum! That forum also contains a changelog.