Could an old injury of my left ankle which has instability and developed an antalgic gait contribute to developing bilateral plantar fasciitis and what rational would there be?

you have developed into an abnormal gait when that happens anything is possible. You would have to resolve the ankle problem if it was causing you to have an abnormal gait thus causing planter fasciitis. you can try supports or custom orthotics from a podiatrist to treat biomechanical overuse problem

Any change in gait can effect the low back,Hips,Knees,legs, and foot. We are designed as a closed Kinetic chain . (only as good as the weakest link. My answer is yes it can cause plantar fasciitis.