JShelter isn’t mainly for spoofing, it’s about blocking a bunch of potentially harmful advanced javascript features, often used for tracking. Any spoofing is mostly to keep sites working with the missing javascript features.
I have it installed on one of my browsers. I wouldn’t recommend using it unless you’re willing to tweak the settings for new sites you visit, because I’ve had it break sites pretty often with the default settings.
There was also pinetta, although the code hasn’t been updated in a couple years.