gonna repost a twitter thread i did a while ago, just in case something happens to that thread. i want to add more details sometime, but this will do for now. maybe one day, I'll rewrite my thesis defense presentation in English and post that to my website, but today is not that day
Over 120 years have passed since the JP government made an order in 1900 for primary school textbooks to only use one type of hiragana character per syllable. But that order was only about textbooks—what about other books?
Turns out that the use of "hentaigana" (non-standard kana) dramatically decreased in movable type printed books around 1890—ten years prior to the government's order. I did a paper about this. It's in Japanese but it has nice charts on p. 29: https://da.lib.kobe-u.ac.jp/da/kernel/81011665/81011665.pdf (the url for the paper changed since I made the twitter thread, and i have now corrected it. Thanks to @luumi for letting me know about that!! if the url ever changes again, search for the title 明治前期の出版物における平仮名字体の使用傾向について on https://cir.nii.ac.jp/ or wherever you might search for papers written in japanese)
Look at Fig. 1 above for woodblock-printed books (X axis is "year of publishing (in the Meiji period)," Y axis is "number of different hiragana characters used in that book"). Woodblock books have a wide spread in terms of the number of different hiragana forms used. There were fewer woodblock books published after 1882 so I stopped collecting data from there on.
Next, look at Fig. 2 above for movable type books (same axes as Fig. 1). There are outliers, but you can see a clear downward slope. There is also a clear-ish cutoff at M. 21 (1888) where most books have under 65 types of hiragana characters (remember that there are 48 hiragana "sounds")
I think it is rare for (non-violent) "top-down" approaches to succeed in terms of language change. The standardization of hiragana in textbooks was not actively trying to restrict hentaigana use (as people may commonly believe); it was actually following in line with common practice within publishing at the time.
In a later study I focused on books printed by different companies but sold by the same publisher around the year 1890—I found that the use of hentaigana differed depending on the printing company (more specifically, the person listed as the representative of the printing company). There is clearly something going on at type foundries and printing companies around this time, but I was unable to identify more details. (i wish i could find the answer, but haven't looked any further since I finished my thesis, and I don't do any research anymore... i hope that I can stumble across some kind of clue one day, but i'm not gonna hold my breath)
It is great that these books list the person and company who printed them—this was mandated for books published in 1888 and onward in Japan. Sadly, older books rarely list the printer, making it harder to further investigate the relationship between printer and hentaigana usage
