[smarinov]

Yes, I plead guilty and am utterly ashamed of myself. My usage above was totally misleading when we consider the way people write nowadays. :)

In Bulgaria, the letter was first briefly removed in a reform between 1921 and 1923 and then removed by the new regime after the end of World War II. It was surrounded by controversy for a long time because in general there is one single way a word can be read in Standard Modern Bulgarian–save for the location of the stress–, which was not the case for this letter. In addition, prior to its official introduction after Bulgaria regained autonomy–and then independence–there were multiple ways to denote the sounds it used to stand for.

But for the sake of etymology I find it quite useful in a lot of places since it pops up in common but generally regular vowel changes across Slavic languages.

[pvg]

in a reform between 1921 and 1923

Wait, really? Was this some zany БЗНС/Стамболийски thing?

[smarinov]

Yes, it was a БЗНС thing at first. There were three major reforms in the way Modern Standard Bulgarian is written:

0. no official codification (before 1899; basically everybody wrote as they pleased and there were some differences between various authors); at some point there was a mostly standard way to write in Bulgarian, though, which was introduced and used by the precursor of the Bulgarian Academy of Sciences and various other institutions; https://bg.wikipedia.org/wiki/Дриновски_правопис

1. first official codification (1899–1921 and 1923–1945 with some minor changes); https://bg.wikipedia.org/wiki/Иванчевски_правопис

2. shortly-lived attempt to modernise it (1921–1923); https://bg.wikipedia.org/wiki/Омарчевски_правопис

3. last and so-far successful modernisation, since it wasn't succeeded by anything else yet (1945–nowadays). It was partly inspired from the previous attempt and partly by Lenin's reforms in Russian from 1918, and not from other Bulgarian or e.g. Serbian attempts, for whatever reason. https://bg.wikipedia.org/wiki/Правописна_реформа_на_българск...