git blame suggests the change was made 15 months ago, though that doesn't take it to account time to publish.
Which I guess just goes to show that it can be tough to find information on the docs, despite them been relatively decent as far as docs go (in my opinion).
https://docs.aws.amazon.com/elasticloadbalancing/latest/appl...
Right at the top of the page, underneath the header "Limits"
> The maximum size of the response JSON that the Lambda function can send is 1 MB.