How do I get old version of Premiere pro? Ideally V21

you're looking for #3:

1. you can install the latest and 1 version prior versions using the cc desktop app. you must have a subscription because previous versions are not available with trials. other versions are revealed after clicking file preferences>apps>tick show older apps (at panel bottom).

2. for versions cc 2019 and older, use prodesign tools.com*.

3. for versions newer than cc 2019 but older than one previous version, contact adobe support for links.**

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* most (but not all) adobe download links are available from prodesigntools.com. https://prodesigntools.com/adobe-direct-download-links.html

follow the directions (esp, step 1)

save the install file. it may not remain online.

thank jim @ prodesigntools.com
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** there are 3 ways to contact adobe; chat, phone and twitter:

chat:
use a browser that allows popups and cookies, and click here, https://helpx.adobe.com/contact.html?rghtup=autoOpen
in the chat field (lower right), type AGENT
be patient, it can take quite a while to reach a human.

phone:
https://helpx.adobe.com/contact/phone.html

twitter:
tweet @AdobeCare

p.s. if you're contacted by anyone (via email or private message), it's much more likely to be a scammer than an adobe representative. ie, double check for an employee badge if contacted in the forums and look for an adobe.com domain in the email address if you click reply to an email. then check again and be very suspicious. any other method of contacting (or offering to contact you) is almost certainly a scam,

https://community.adobe.com/t5/using-the-community-discussions/adobe-support-community-guidelines/td-p/4788157#:~:text=constructive%2C%20and%20encouraging.-,3.%C2%A0Be%20Safe%C2%A0,-Do%20not%20share