Even if one stopped using Unity to develop new things before this change, they are still on the hook for product installs (even if they are free games), which are by the way tracked by Unity "proprietary data model".
Example: https://www.reddit.com/r/Unity3D/comments/16hgmqm/unity_want...
A few quotes from the FAQ:
Q: If a user reinstalls/redownloads a game / changes their hardware, will that count as multiple installs?
A: Yes. The creator will need to pay for all future installs. The reason is that Unity doesn’t receive end-player information, just aggregate data.
Q: Are these fees going to apply to games which have been out for years already? If you met the threshold 2 years ago, you'll start owing for any installs monthly from January, no? (in theory). It says they'll use previous installs to determine threshold eligibility & then you'll start owing them for the new ones.
A: Yes, assuming the game is eligible and distributing the Unity Runtime then runtime fees will apply. We look at a game's lifetime installs to determine eligibility for the runtime fee. Then we bill the runtime fee based on all new installs that occur after January 1, 2024.