I have not had the motivation to maintain these tools for quite a while now, leaving packages to fall very out-of-date. It's possible I may bring SDSetup back one day if I get the itch, but no promises, and certainly not any time soon. For now, it's best you switch to a new source.
Right now, your best alternative is to follow switch.hacks.guide. This guide will walk you through rolling your own Switch CFW setup, including obtaining all files from their original sources. This is the best option to learn about what you are installing on your Switch, the guide is excellent, and the folks at Nintendo Homebrew have a great team of helpers if you get stuck.
If you want a drop-in replacement which provides a ZIP that you just drag and drop, check out Team Neptune's DeepSea package. This is a spiritual successor to Kosmos and should work as you expect.
If you would like to host your own instances of these tools, the source code will remain archived on GitHub
SDSetup • PegaScape • homebrew.guide
(but really, SDSetup should be rewritten from scratch)
Please note that while code and content for SDSetup, homebrew.guide, and PegaScape are released under permissive licenses, logos and graphical assets are copyrighted. If you want to host a public replacement, just change the names and you are good to go!