Can I still use an existing editable form created in Adobe Acrobat after subscription has ended?

You can open the file in Reader, view it, fill in the existing fields and save it, but you can't edit it, or import/export form data (unless you use a script), etc.

Yes, you can use the form in Acrobat Reader.

No, without an active and paid subscription of Adobe Acrobat Pro DC you won't have access to the premiun editing features.

Here's some relevant information in case that you've cancelled your subscription and wish to renew:

• https://helpx.adobe.com/manage-account/using/renew-restart-extend-membership.html