Found this by using VSCode to edit the config file, which automatically puts trailing commas when formatting.

trailing commas aren't allowed in json. the parser (not my code) is just following the spec

https://www.json.org/json-en.html

whatever vscode plugin you have is doing it wrong

You are using json5 though.
It also has a spec. https://spec.json5.org/
And it allows trailing commas. Maybe there's a different parser for it.

I added a clarifying comment to the file:

// NOTE: This is not actually json5 - it's parsed with minecraft's built-in json parser but with
// some preprocessing to allow for '//' -style comments like this one.  The lie in the .json5 
// extension just helps text editors deal with it.