In Product listing page/Category page:

Filter Availability - let's change the value: None into All - which should display all available, and unavailable products (including products without availability defined)

Alternatively, natalia.becla@ibexa.co and dariusz.szut@ibexa.co could we use option 2) and by default (before the buyer selects "available" or "unavailable") display all products, and then this 3rd value in the Availability filter would be not needed?