If you still have a legal license for CC 2015 (e.g., a perpetual license from before Creative Cloud), you can run it on a modern Mac only via virtualization:
This is legal if you own a license. Performance will be poor for video editing, but it works.
Adobe does offer older versions – but only to active Creative Cloud subscribers. Here is the legal process: download adobe premiere pro cc 2015 for mac free
Important caveats:
Cost : At least $20.99/month – not free, but legal and safe. If you still have a legal license for CC 2015 (e
Free trial
Adobe Premiere Rush
Educational or enterprise licensing