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Once upon a time, every single Russian word had to end with either a vowel, a soft sign (ь, мягкий знак) or a hard sign (ъ, твёрдый знак). This was a leftover from when the latter two were pronounced as very short instances of the vowels i and u respectively, and no consonant could end a word; but since in the meantime they had become diacritical signs telling the reader whether the preceding consonant was soft of hard (and word-ending consonants had reappeared by virtue of the muting or almost-muting of these extra-short vowels), it was decided (as part of the 1917 reform IIUC, when my late grandfather was a teenager) to remove the hard sign in that position. It only remains where a prefix ending in a hard consonant is followed by a yod, e.g. in the verb съесть (to eat, perfective, ← с + есть).
I wonder if this dictionary also uses other letters long disappeared from current orthography. For instance, does it spell "мѣждународный" with ѣ, as under the Czars? (In modern spelling, the synonym pair would be международный = интернациональный, "international".)