Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
I can confirm that the V2 API authentication endpoints will continue to function past the end of this month. That being said, while our V2 API endpoints are currently functional, they are considered legacy and we do plan to deprecate them. I would highly recommend migrating your application to our V3 API which I am linking quick start and migration guides for below.