I was surprised to find that there are actually a couple of choices (at least) in this space.
SQL Fiddle is ad-free and support by a .Net services company.
DB Fiddle is ad-supported along with some recruitment sidelines. It has a much more conventional UI and feels much clearer on how you're meant to use it.
DB Fiddle (UK) has a clearer interface for breaking up individual steps, almost like a data science notebook. It is also ad-supported.
All three support a range of databases that is much wider than my needs.
It is great that there is such a degree of different approaches to choose from.