How can I save an Xcode project in Sequoia writen in Xcode Sequoia - Version 15.1 so that it can be opened in Ventura - Version 14.3.1 (14E300c) or in Big Sur Version 13.1 The error I get says Project.xcodeproj cannot be opened because it is in a future Xcode project file format. Adjust the project format using a compatible version of Xcode to allow it to be opened by this version of Xcode
Download Old version of Xcode
If I understand your question, it is not about downloading an old version of Xcode, but having a project opening in an older Xcode version.
To do so: in Xcode,
- select the project in the navigation panel
- open the Inspectors
- in Identity and Type inspector, you can select the project Format.
Note: I have not tested if you can downgrade to older version.