> SQL Server and Oracle both default to heaps

I don't think that's true for SQL Server.

If you define a primary key in SQL Server, this is automatically a clustered index.

Any table in SQL Server without a clustered index will be stored as a heap.

https://docs.microsoft.com/en-us/sql/relational-databases/in...

Yes, but the _default_ (and that's what I was referring to) is a clustered index.

Then the definition depends on whether you consider adding a primary key as default or not. I would not consider it default, though it is customary.