The Decree on Women’s Healthcare that went into effect in 1920 officially and undeniably legalised abortion.

http://soviethistory.msu.edu/1917-2/the-new-woman/the-new-woman-texts/on-the-protection-of-womens-health/

It explicitly states it is legal and available in soviet hospitals:

But as the moral survivals of the past and the difficult economic conditions of the present still compel many women to resort to this operation, the People’s Commissariats of Health and of Justice, anxious to protect the health of the women and considering that the method of repressions in this field fails entirely to achieve this aim, have decided:

(1) To permit such operations to be made freely and without any charge in Soviet hospitals, where conditions are assured of minimizing the harm of the operation.

(2) Absolutely to forbid anyone but a doctor to carry out this operation.

(3) Any nurse or midwife found guilty of making such an operation will be deprived of the right to practice, and tried by a People’s Court.

(4) A doctor carrying out an abortion in his private practice with mercenary aims will be called to account by a People’s Court.

I think the correct interpretation of this poster is that it is against abortions performed by anyone other than a DOCTOR in a soviet hospital.