I was certified through my employer (via an ACP), but have now left the company. Can I keep my certification?

Yes, absolutely.

To let the Certification Authority know that you’ve changed employers, Update your Profile with your new business email address and employer name.

Further information can be found in Section 7, Change of Employment, in the Open Professions Certification Policy: https://certification.opengroup.org/docs/Open_Professions_Certification_Policy.pdf