These kind of announcements are also referred to as “nag notices” - most wordpress themes, or PHP scripts might have them. It’s a way of reminding the buyer to either renew support (as in this case) or to update a theme/plugin if it is out of date.
First of all, your theme will continue to work even with this notice. Nothing will change for the visitors of your website with our theme, even when your license expires.
There is a way you can disable this notice by editing the functions php file, or better still contact your seller and ask them.