You can find json files on the webserve for specific release trains:
https://www.haproxy.org/download/3.4/src/releases.json
You may also just use the external service endoflife.date :
https://endoflife.date/haproxy
Using their API you can use the one liner:
curl -s "https://endoflife.date/api/v1/products/haproxy/releases/latest" | \
jq '.["result"]["latest"]["name"]'