[breezeTrowel]

This is retroactive. The license is accepted via the Unity engine. The games include a Unity runtime and this fee applies to new copies of all existing games. How is this not retroactive? The games were bundled with the runtime in the past. Plus the new license applies to all previous versions of the Unity engine that have already been installed. This is 100% retroactive.

[angry_octet]

They say that the new licence does not apply to existing installs. However, those installs are counted for determining if you cross the new sales threshold. That is a prospective change.

You're complaining that the licence for runtime instances should be locked to the one that was in existence when you wrote the game (using the editor). This is entirely reasonable, but not a right you have under the old or the new licence. They could change it again!

[breezeTrowel]

You had that right under the old license before it was changed and the repo tracking the ToS was deleted. This is a retroactive change. If people are downloading your installer that was published a year ago, people running that installer after January 1st will now cost you money.

[angry_octet]

The license text being deleted doesn't change the contract, it is still extant, and hence nothing retroactive about it.

You should absolutely disable installations in the future. Pull all binaries from the web and replace them with ones that require an activation key.

If you have made a best effort to comply with the new terms, they cannot come at you for people running an old installer.