They have a civic responsibility to flag lies. They repeatedly warned liars of their lying, flagging them with "read our freaking rules about lying', then at some point they punish the violators by banning them.
If the Russian and Chinese governments were endlessly lying about America on social media, would you decry their getting banned by social media?
Plenty of conservative voices are alive and well on social media. Jonah Goldberg, Ann Coulter, Bill Kristol to name a few off the top of my head. While I mostly don't agree their opinions, I at least concede that they are not liars.
To conclude, there is no concerted effort to stifle conservative free speech on Social media. There is a focused effort to stop lying and the spread of falsehoods.
Lying conservatives are free to create their own lying social media to sow their lies, as far as I know. It's still a free country.