If I forget to extend my XenServer 7 license in time, would my virtual machine be deleted?

No, neither the virtual machine nor the data on it is lost. However, the status “Unlicensed” would be displayed in the XenCenter License Manager. In addition, the corresponding features can no longer be used until a new license is purchased.

Sources:

Official Citrix XenServer 7.0 installation guide_as is 2016-08-25