The Russian language has never completely eliminated the "h" sound. The sound represented by the letter "х" in Russian is called the voiceless velar fricative, which is similar to the "ch" sound in the German word "Bach" or the "ch" sound in the Scottish word "loch."
However, there have been some historical changes in the pronunciation of the "х" sound in Russian. In the 15th and 16th centuries, the sound was pronounced as a voiceless glottal fricative, similar to the "h" sound in English. This pronunciation is still preserved in some dialects, particularly in certain regions of Russia.
During the 18th century, there was a linguistic shift in the pronunciation of the "х" sound. It gradually transformed into a voiceless velar fricative, which is the current pronunciation in standard Russian. This change is known as the "guttural shift" and is reflected in the modern pronunciation of words like "хорошо" (khorosho, meaning "good") or "хлеб" (khleb, meaning "bread").
So, while there has been a historical change in the pronunciation of the "х" sound in Russian, it hasn't completely disappeared. The modern pronunciation of the "х" sound is still an important part of the Russian language.