The install page Install /e/OS on a Nothing - Nothing CMF Phone 1 - "tetris" | /e/OS documentation does not tell us a specific Android version required before install of the current /e/OS a14 build.
The Fastboot ROM provided is expected to be capable to change Android version. Here, for instance, Rollback android 15 to android 14 - #20 by circonflex is a report on downgrade from a15.