Did they cite a reason that sounded reasonable? Like the particular implementation breaks some design principle they want to stick to or something? Did they suggest it might be acceptable some other way or in some other form?
It's curious because gawk cannot for one second claim something like needing to stick to some legacy standard, not with a straight face.
If I were the gawk maintainer I would be unwilling to take on features by default. It is widely used infrastructure and keeping out bugs is far more important than taking on features. If the userbase keeps asking for the same feature over and over, at that point it would be up for consideration, but no sooner.
There is more around it. I had the idea in two other forms.
Initially I had a @param:<ident> syntax which indicated that the given variable is to be allocated in the parameter space (a local variable frame where function parameters go). This only worked inside functions.
Between that and @let was a @local thing.
The maintainer of GNU Awk is one of the two authors of the "Fork y Code Please", the other being the Bash guy:
I patched GNU Awk to have a @let extension that gives you scoped locals (usable in functions as well as in BEGIN/END blocks):
@ is used because there is at least one other existing extension which is like that: @include.https://www.kylheku.com/cgit/egawk/about/
This was rejected by the GNU Awk project, though. I was encouraged to make a fork and give it some kind of different name, so I did that.