You are correct: on December 27th, 1917, Lenin abolished all Imperial ranks, titles, orders, and other marks of distinction. This included the St George's Cross and Medals, which never again came up in the manuals on the order of wearing decorations. Officially, their wearing was forbidden.

However, the St George's awards were special, since they were awarded only to soldiers and didn't have the "taint" of the officer class associated them. Many soldiers continued to wear theirs unofficially as a result. This was tolerated, and many high ranking officers are seen in photographs wearing theirs underneath their Soviet medals, where medals from foreign nations are traditionally displayed. This was not always the case, for instance Budyonny is sometimes photographed with his St George's Crosses and sometimes without.

While tolerated, the St George's Cross was never officially recognized by the Soviet government. There was a petition to equate the Cross with the Order of Glory, and even a draft GKO decree was penned to this extent, but it was never passed.

Sources:

https://www.cia.gov/library/readingroom/docs/CIA-RDP73B00296R000100120004-4.pdf

«Военно-исторический Журнал» № 5, май 2005, с. 69

Приказ №240 НКО СССР "О правиле ношения орденов, медалей, орденских лент, лент медалей и военных знаков отличия военнослужащими Красной Армии"