Chris,
You can just remove the ArchiCAD 8 folder from your applications folder. However, remove the library folder first and keep it so your older projects can use the old library. Don't bother trying to clean up your old preferences. They don't take up much room and some of them will have the AC10 preferences mixed with them.
Also, if you haven't been doing this go to your Library/Caches/Graphisoft/ArchiCAD Cache Folder AC8. folder and delete the several hundred meg worth of Scratch files. Also do this on occasion in the AC 10 folder. This is Graphisofts most egregious programming error in my opinion.
Mac OS12.6 AC26 USA Silicon
M1 Macbook Pro