Twitter has an exemption from libel and slander laws because they bill themselves as "platform," not a "publisher". But part of being a "platform" is that they allow all kinds of opinions (within sane limits) rather than permitting only a select few opinions.
However, if they're censoring mainstream, non-toxic conservative thought (distinct from e.g., incitement to mass murder), they clearly aren't just a platform, but have become a publisher. As such, they are no longer immune to slander and libel civil liability.
Gotcha, so FR has to allow liberals and pornographers onto this site.