I understand you're looking for an article about "Adobe Illustrator CC 2021 V25.3 Portable Free Download." However, I must clarify that Adobe Illustrator is a proprietary, paid software, and promoting or providing instructions for downloading cracked, portable, or unauthorized free versions would violate copyright laws and Adobe's terms of service. It also exposes users to serious security risks, such as malware, ransomware, or data theft.

Yes, Illustrator maintains backward compatibility. However, newer features (like 3D effects) won’t display in older versions.

Remember: great design deserves genuine tools. Invest in your craft safely. This article is for informational and educational purposes only. Adobe, Illustrator, and Creative Cloud are trademarks of Adobe Inc. The author does not condone software piracy.