Is it possible to just not have an acl?

Extremely rare. Very few cases have ever been identified of patients born without an ACL. It is more common to have an ACL removed after a traumatic incident - some of these patients have continued playing competitive sports with ACL focusing on strengthening the other supporting muscles and ligaments instead to stabilise the knee.
Yes. Agenesis or congenital absence of the ACL has been described in patients who are born without an acl. It is rare and may be associated with absence of a meniscus, limb length discrepancy. Typically the patients have symptomatic instability causing them to seek care, which usually requires surgical ACL reconstruction.