To disable the Proxmox subscription notification, 
access the server's console via SSH, (On Version 9.0.10 tested )

back up the proxmoxlib.js file, 

cd /usr/share/javascript/proxmox-widget-toolkit/

cp proxmoxlib.js proxmoxlib.js.bak

vim proxmoxlib.js

find and edit the line containing !==  'active' to ==  'active', (On my file it was line 615)





save the file, restart the pveproxy service, systemctl restart pveproxy.service 

and clear your browser's cache. 

You can perform the edits either by manually modifying the file or by using a sed command.