It depends entirely on the source.
Inside the Creative Cloud app, click the three dots ( ... ) next to Illustrator and select "Other Versions." Download Adobe Illustrator 2021 zip
Downloading a pre-activated .zip of Illustrator 2021 from torrent sites, file-sharing forums (like RuTracker, 1337x, or others), or random blogs. This violates Adobe’s EULA and copyright law. It depends entirely on the source