What work now: disable System Integrity
Restart your system in Recovery Mode (depends on each mac)
On terminal (accessible via utilities), run csrutil disable
Restart the mac and you will be able to open the old Xcode just like the new one.
Right after using the desired Xcode version, reenable with csrutil enable , as disabling it makes your system more insecure.
(All previous versions stopped working on Sequoia 15.4.1+)