Hey Sophie,
When reviewing S1610, we realized some conceptual problems with this rule. Basically, default interface methods are just there to have multiple inheritance also with method implementations, not only API definitions. I.e., an abstract class can implement methods, but you cannot inherit from multiple abstract classes. Default methods solve this problem but at the cost of mixing interface and implementation.
Developers should, therefore, not be forced to replace abstract classes with interfaces and default methods. Maybe they want this clean separation between API and implementation. It should be their design decision.
There are also other possible reasons why a developer might want to keep an abstract class, for example, when they work with tools or frameworks that require it to be a class or to provide an extendable class in a public API with a clean inheritance hierarchy.
Best,
Marco