Unless something has recently changed I don't believe ba.com provides an expiration date for avios (at least I've never found one). Dates for membership year and card expiry are provided. Each member is responsible for tracking expiration based upon their last activity.
https://www.flyertalk.com/forum/british-airways-executive-club/2079328-verify-avios-expiration-date-website.html
https://www.flyertalk.com/forum/british-airways-executive-club/2101609-avios-expiry.html