It's not, and it won't be for probably a while; I don't believe video filters have been implemented at all yet, and there are higher-priority things that need implementing in the meantime.
The devs know that this is an important feature for people, and it's certainly planned, so please be patient. In the meantime, I believe CamTwist has a chroma-keying capabilities, and can be added as a video device into OBS-Studio, so you could pipe your camera through that.
