nano /usr/share/pve-manager/ext4/pvemanagerlib.js
Currently on line 519 of the file, however it may change with future updates, there is a line similar to below;
if (data.status !== 'Active') {
This line is doing the check to see if your subscription status is not ‘Active’. This needs to be changed to return false to stop the subscription message from being shown.
if (false) {
And that annoying little popup will be a thing of the past!
Note: You may need to clear your web browser cache after applying this code change.