Before discussing the "Portable" aspect, it is important to understand what makes this specific version (V25.3) technically significant:
No. Adobe does not release portable versions of any Creative Cloud app. Portable versions require reverse-engineering the software to run without installation, which violates the license agreement. The only official way to run Illustrator without permanent installation is via (limited functionality) or using a virtual machine with a licensed copy installed—but that’s not truly portable. Adobe Illustrator CC 2021 V25.3 Portable Free Download
The June 2021 update focused on enhancing creativity and streamlining workflows with these primary additions: 3D Effects and Materials Before discussing the "Portable" aspect, it is important
To run Illustrator CC 2021 smoothly, your system should meet these specifications: Illustrator Cloud Documents & the Version History Panel Before discussing the "Portable" aspect