If you enable this policy InternetExplorerIntegrationLocalSiteListExpirationDays
, you can keep the IE mode sites for 90 days at most.
If you don't want your IE mode sites to expire, you need to configure IE mode sites through policies. For the detailed steps, you can refer to this doc. There're two steps required to configure IE mode:
After doing the above, the IE mode sites will never expire unless you changed the policies.
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Regards,
Yu Zhou