openSAP is open to anyone in the world so it doesn't matter if you have left SAP. However, please make sure that you change your email address under your openSAP profile to your personal email address. This email will be used to send you notifications and will be included on your openSAP Record of Achievement.
If you have questions regarding how you should change your email address, please raise a ticket with the openSAP help desk.
I hope that helps and best of luck in your future career.