The Orthodox Church is very open and active in Russia, so obviously Christianity has not been outlawed. However, I do know that in the past, Protestant and Catholic Christians have faced some restrictions, and some persecutions, with the blessing of the Russian Orthodox Church. From what I gather, one reason for Protestants and Catholics facing restrictions is that they are regarded as foreign influences. For example, in the past, Baptists were persecuted because much of the Baptist missionary work in Russia was financed by John D. Rockefeller. I can’t say what the situation is now but, if there are restrictions, past experience can probably tell us why.
I write this as an Orthodox Christian who is often very unhappy with my brethren in Moscow.
They are persecuted because it would be living and passionate Christianity instead of a birth and marriage ceremonial culture church controlled by a repressive government, similar to how China handles Christianity, persecute and suppress the committed Christians while giving the the people a government approved and connected version to handle the visuals and ceremonies to absorb the Christian impulses.