FYI: Mostly a reminder than issue since these files are often forgotten to be updated. There are files to change for 1.17.1 to work.
mods.toml, mixins and pack.mcmeta need updating for every major release of Minecraft depending on what is included.
Minecraft 1.17.x - Minimum Java Level Supported: 16.0.1
pack.mcmeta files Pack Format=7 for 1.17.x release, 6 for 1.16 and 5 for 1.15, etc.
Minecraft Vanilla Launcher (new one from Microsoft Store) has loading issues that require additional changes to our mods and modpacks. Read conversation towards the bottom on Forge Support: https://forums.minecraftforge.net/topic/105079-forge-1171-not-launching/?_fromLogin=1
This describes how to workaround the breaking change Microsoft made with the new Vanilla Minecraft launcher. I'm posting here as I've looked at this 1.17.x version of the mod and found one or both issues exist. It wasn't an easy process figuring out why mods were not working properly so I've provided the data so you don't have to waste time trying to figure out why.

Changing the minimum jvm compatibility version and the pack format aren't going to change the mod's ability to load (packs can still be loaded from older versions just fine, and older jvm compat limits are irrelevant since 16 > 8), and placebo doesn't have any mixins.

Also, that link doesn't describe anything relating to working around issues, it's just someone trying to run the game on java 17 which is unsupported.