When the Forever option is selected for suspending a user, the user is suspended for 1000 years. Without customizing the site’s text, this time period is displayed to the user in the suspension email that is sent to the user, and if the user attempts to log back into the site.
I believe that the suspend time period used to be shown for permanently suspended users on the user card, but that the time period is no longer displayed there when a user is suspended forever. I think that is related to the changes that were discussed here: Clearer display of "forever" suspensions/silencing.
Could all mentions of a user having been suspended for 1000 years be removed from notifications and user facing text on Discourse? Instead of doing that, just let the user know that they have been permanently suspended from the site. Telling someone that they have been suspended for 1000 years seems likely to come across as a bad attempt at humour.