Lightroom classic

The Creative Cloud desktop app provides only the last two major versions (currently its version 15 and 14).

Contact the Adobe customer support and ask to provide download links to earlier versions. Explain why you need them.

Click on the chat bubble in the lower-right corner: https://helpx.adobe.com/support/lightroom-classic.html

Type "agent" to chat with a human. If the agent isn't able or doesn't want to provide you with a download link, hang up and try again. Some have reported trying several agents before getting one who could help.

If you get the appropriate link and after you've download the installation file it is recommended to save this file to a save place, like external hard drive or usb stick, for a further usage. It isn't guaranteed how long Adobe will make such versions available on demand.