Any future installs starting on January 1. It does, however, mean that many developers will be more or less forced to pull their games off of storefronts, if it actually goes through. It also means that if you bought a Unity game in the past, you’re costing the developer money every time you install it (again, if this actually goes through - I can’t imagine they won’t backpedal.)
The real issue with this isn’t the policy itself, which I would bet money won’t actually be enacted, but the fact that Unity (thinks they) can just unilaterally and retroactively change their policies. If this actually held up in court, which I think is a tenuous possibility at best (but I am not a lawyer so take that with a grain of salt), it sets an awful, awful precedent.