Google deletes legacy Google Apps domains if none of the accounts at that domain are logged into for a year or something.

I've received multiple warnings for some of my unused domains.

This I believe only applies to the root/admin account of the GSuite domain. Extra users created in the account are just extra user slots for that account.

I usually don't use the admin account as my main working account, for security.

I believe the Google Apps "target for deletion" process only gets triggered if none of the accounts in a domain are logged into for a period of time.