Yet the default Godot download is a 'standard edition' without Mono support?
What's the reasoning behind the two separate builds? Is it just about build/download size, or are there potential licensing issues with Mono or something?
After looking around the web, it seems the version that contains the .NET (mono) is about twice the size. It looks like the Godot team maintains two package versions simply to avoid bloat if you do not require that functionality.
Godot's system for exporting games/apps/projects as binaries is that the same codebase/build system that generates the editor binary (the Godot "tools") can be built with the "tools" parts omitted, and the resulting non-tools release binary is bundled up with your project scripts/resources as the binary API for your exported game.
The general philosophy for Godot's build/compilation system is that most things are just linked statically, and features and modules can be turned on and off via build flags, so that they can be as portable as possible - indeed it's usually pretty painless and quick to export a game to a binary from the editor on most platforms because of this.
With Mono there are 2 sticking points that make this a bit more difficult - first is that this non-tools binary has to bundle the Mono runtime with it to be portable, this adds a bunch of bloat, and Godot's style has always been to be as light as possible. For mobile apps for example, that extra ~40mb of stuff can be a real problem.
Additionally, the development machine has to have a fairly modern dotnet SDK installed (ideally .NET 6 SDK + dotnet cli these days) as well as the app host targeting packs in order for the mono-enabled editor binary to run at all (as it contains some CLR/native interop code and nuget packages for the editor tools to copy into your project, etc that fails without these things). So the export template (the non-tools godot API binary) with Mono can't be understood by the editor tools without Mono enabled.
Recently direct .NET 6 interop support has replaced the Mono integration in Godot 4, and this might in the future change the above - since it will be able to use the new AOT stuff to publish exported games in a more direct way. It's definitely a stated intention to eventually ship just one engine binary for everything, but it's still a way off happening yet, maybe in Godot 4.1 or 4.2.
What's the reasoning behind the two separate builds? Is it just about build/download size, or are there potential licensing issues with Mono or something?