Abridged history of spaced repetition
1885: Hermann Ebbinghaus plots the first forgetting curve. Although it didn’t have retention on the Y axis, and also, if you have ever seen one of the images below (or something similar), you should know that his paper didn’t have that serrated kind of curve. That is a common myth.
1885-1972: nothing. Some researcher occasionally publishes a paper about the spacing effect, which nobody cares about. I wouldn’t even be surprised if multiple researchers re-discovered the spacing effect independently.
1972: Sebastian Leitner invents the Leitner system. As crude as it is, it’s the first spaced repetition system that looks like what spaced repetition looks like today. Learning steps in Anki are essentially that.
1985: SM-0 is developed. It wasn’t a computer algorithm, and was done purely with paper notes.
1987: SM-2 is developed, it is still used in Anki and other apps, like Mnemosyne.
1987-2010s: not much. Piotr Wozniak develops SM-5, SM-whatever, but they are proprietary, so this has little to no impact on spaced repetition research and other apps.
2010s: Duolingo develops HLR. Some other models, like ACT-R and DASH are developed by other people, but nobody gives a damn. To the best of my knowledge, neither ACT-R nor any of the DASH variants have ever been used outside of a scientific paper. Woz develops SM-17 and SM-18, they are also proprietary. However, he does describe key concepts and ideas on supermemo.guru, which was important for developing FSRS.
2022: FSRS v3 is developed. This was the first publicly available version that people actually used. FSRS v1 and v2 weren’t publicly available.
2023: For the first time since the development of SM-2, app developers start implementing a new algo - FSRS, with Anki being the pioneer. Though it’s possible that some obscure app has experimented with machine learning (excluding Duolingo, I have already mentioned them) and I am simply unaware of that.
2024: RemNote implements FSRS-4.5 (or FSRS v4? I’m not sure), some chess moves learning app apparently does too.