It would be better if Outsytems provide a Notification/Warning over Service Center/Lifetime of On-premises platforms if they are using/having an older Version of On-premises Platform Server to upgrade to higher/latest version and utilize new functionality/features that latest version having.
It is not mandatory to upgrade but it would be better to aware users. If they are seeing warning/Notification, they might get aware and do platform upgrade.
Notification/warning should be start showing if they are using Platform server 10/15 version prior to latest one.
The only reason It is always a good practice to have latest version deployed and in use.