The Russian orthographic reform enacted in 1917 and 1918 removed four letters from the alphabet and changed the rules for a fifth. The decimal i, yat, fita and izhitsa were abolished, and the hard sign was restricted to a single function inside words, ending the practice of writing it at the end of every word that closed with a consonant.
The reform is often remembered as a Bolshevik measure, and the decrees that enforced it were. The proposals were not. They had been worked out by a commission of the Academy of Sciences and published before the revolution, and every letter removed had been argued about by philologists for decades on the ground that it marked no distinction any Russian speaker made. What changed in 1918 was not the linguistics but the willingness to impose it, and the enforcement was blunt: printing houses had the offending sorts of type physically removed, which is why some early Soviet books use an apostrophe where a hard sign should stand.
Two constituencies kept the old letters. The church continued, and continues, to print its Slavonic books in the old orthography, so the letters are not obsolete in the sense of being unreadable. And the emigre presses kept the old spelling for decades as a deliberate refusal, which turned an argument about redundant vowels into a marker of political allegiance.
Bulgaria carried out a comparable reform in 1945, and it removed a different set of letters, because the two languages had lost different distinctions.