While the Soviets held substantial influence over many communist and socialist movements, there had been opposition to Stalin and his particular brand of communism from the start.

First of all, there were internal opposition, mostly in the form of Trotkiy and his supporters, which eventually got Trotskiy exiled and later murdered in Mexico.

Secondly, there were plenty of socialist and communist movements that were anti-Stalinist. The social democratic parties of the Nordic countries were socialist movements that rejected a communist revolution, Stalin and the Soviet Union. Likewise, there were movements of syndicalists and anarchists that opposed Stalinism, both in the Soviet Union (they were crushed during the civil war) and outside it. The Spanish anarchists of the CNT-FAI and the Soviet-loyal communists of the Spanish Republic were uneasy allies that actually became full-on enemies.

The Soviet Union could boast of a large amount of prestige after ww2 - no-one else had bled so much and fougth so much to defeat the nazis. Western propaganda had soothed ruffled feathers and excused, downplayed or downright denied Soviet crimes (such as the Katyn massacre) in the interest of the war.

In the 40s, the Soviet-loyal communists were often the largest and most influential left-wing movement in many countries, so for a socialist to decry Stalin and Stalinism was to risk ostracism from any Soviet-loyal party, event, congress or meeting (which would be most of them at this time).

It was rare, and it would probably be considered pretty daring to do so, but it was not unheard of.

Valentin Gonzales, "El Campesino", republican militia leader and later regular divisional commander in the Spanish Civil War decried Stalinism as "fascism wrapped in a red flag" after the war. So Orwell was not completely alone.

Edit: Changed the rank of Valentin Gonzales as per /u/tobbinator 's correction.