Endpoint Protection definitions out-of-date message cannot be customized in SEP 15 using the cloud management console.
In particular, a SEP client of an older version that was managed with the on-premises SEPM (SEP Manager) may have used a customized definitions out-of-date message and after upgrading the client to SEP 15 it continues using the same message but the cloud management interface provides no option to change it.
SEP 15 client for Windows
Customizing the definitions out-of-date message is not currently a feature of the cloud management interface, but can be done for individual clients by editing the related registry settings:
HKLM\SOFTWARE\... (HKLM\SOFTWARE\Wow6432Node\... on 64-bit machines)
...Symantec\Symantec Endpoint Protection\AV\AdministratorOnly\General\
DisplayOutdatedMessage (DWORD) - 1 or 0
PatternWarningMessage (SZ) - not present if no custom message is set
WarnAfterDays (DWORD) - number of days after which defs are considered out-of-date
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