IIUC the `use` declaration is including the target namespace (https://laravel.com/api/8.x/Illuminate/Support/Arr.html) in the current scope, not declaring a namespace itself. If you wanted to create a namespace you need the `namespace` declaration, which this file lacks. `helpers.php` is directly included by both `index.php` and `artisan` and the functions are thereby in the global scope of the entire app.
In any case, it wouldn't make sense for that file to be namespaced under Illuminate\Support\Arr, as most of those functions have nothing to do with arrays.
Anyway, it _could_ have been namespaced. It _should_ have been namespaced. I'm sad for the app author, but he should have seen that coming. His app was developed well after namespaces were introduced.
You're making impossible demands. PHP has chosen to be backwards compatible _to a certain point in the past_, and it is generous in what it decides to keep working.
To expect that 8.4 will care about not breaking some code that was written with 5.2 style is unrealistic.
You're making a time travel judgement. You're saying that PHP should have renamed everything back then to categorized namespaces and broken compatibility _way_ earlier, which is actually a much worse break than the break that actually happened.
8.4 is not even backwards compatible with perfectly working code written to target 8.x. It doesn't materially matter that the code is written in "5.2" style if it was meant to run on only 8.x. Even that style point is arguable. The author uses Composer! And Laravel! And mostly does use namespaces, outside of a few useful helper functions that are reasonably global so they don't keep needing to import them or use namespace prefixes.
In any case, it wouldn't make sense for that file to be namespaced under Illuminate\Support\Arr, as most of those functions have nothing to do with arrays.