You can legally buy a used CS6 license from original owners (e.g., on eBay or design forums). Adobe still allows activation of CS6 via their legacy servers. Once activated, you can use it offline forever. This is not "free" nor "portable" out of the box, but you can install it to an external SSD and use it on multiple machines (one at a time).
: Adobe offers a 7-day free trial for the current version of Illustrator, providing full access to all latest tools and updates. adobe illustrator cs6 portable multi free
Adobe does not authorize "portable" versions; using them violates Terms of Service. 💻 System Requirements (CS6) You can legally buy a used CS6 license