We should allow trailing comma in function applications universally, not just only variadic functions. This simplifies the grammar, parsing code, and it's going to be much easier for developers to maintain and for users to understand.
This possibility was removed as of T153251. IMHO, it could be confusing to allow trailing commas in non-variadic functions. I'm also not too sure that it will make the code easier to maintain, or easier for people to write filters. All (non-variadic) AF functions take between 0 and 2 parameters, str_replace being the only one which takes three.
I still do think it would be better if the grammar of the language doesn't need to know about tiny details of functions (that is, the ideal grammar should be stable, while it should be easy to add new functions). That said, I don't have strong opinion on this, so feel free to mark it as declined if you want.