Googling the question right now, there are two recent developments I was unaware of. There’s a 2018 paper [1] about it showing no perceptual benefit to either spacing, and also in 2020 MS Word changed their spelling checker to consider 2 spaces an error [2]. This second article tells the opposite story from what I thought I’d heard (but maybe this is what I was thinking of) - it says 2 spaces were developed for monospaced typewriters, and not for print.
[1] https://link.springer.com/article/10.3758/s13414-018-1527-6
[2] https://www.grammarly.com/blog/spaces-after-period/