We are deploying TA924e 3rd Gen IAD. Will /31 bit subnet mask be supported (when port is configured for wan purpose) ? If yes, do I have have upgrade my AOS version to support it? What version?
Yes, it's supported.
I'm not sure of the earliest firmware revision that supports /31 masks but it has been supported since before the 3rd gen TA900 series was introduced so you're good to go regardless of firmware. It's best practice to use the latest extended maintenance release in any case, however.