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...