Is there any reference that can back-up this claim?
Perhaps it's related to Electrum's existing seed phrase version number which indicates which script type the wallet should be.
Implementing a multi-script wallet will make that feature obsolete. But that doesn't mean it's not open for update.

Even one of the main developerdevelopers has this issue opened:
Implement advanced wallet option: multiple xpubs, mixed script types, and similar (maybe with output script descriptors) #6016

Is there any reference that can back-up this claim?
Perhaps it's related to Electrum's existing seed phrase version number which indicates which script type the wallet should be.
Implementing a multi-script wallet will make that feature obsolete.

Even one of the main developer has this issue opened:
Implement advanced wallet option: multiple xpubs, mixed script types, and similar (maybe with output script descriptors) #6016