Michael,
That's certainly worth looking at - given how long ago the change was made, I don't know if the issue persists or if the acceptable delay varies between synthesizers. I would very much encourage you to explore creating an issue. We have recently changed the template to make it easier to file issues. Here is a direct link to the feature request issue template, which is shorter than the bug report template (and which this would fall under):
https://github.com/nvaccess/nvda/issues/new?template=02-feature_request.yaml
All it wants is:
1) Title: a line of text something like: "Add a setting to specify the number of selected characters read"
2) "Is your feature request related to a problem, please describe" - describe the problem (eg how you would like to be able to adjust the amount of selected text NVDA can read before simply announcing the character count)
3) "Describe the solution you'd like" - just as you described, a setting to specify the amount of selected text it will read before giving a count.
4) "Describe alternatives you've considered" - here you could put investigating the ongoing need for a cutoff point (if it's not needed at all, it could just be removed and no setting would be required)
5) "Additional context" anything else relevant.
So there's really only three things you need to provide - a title for the issue, describe the problem and articulate the solution you've got in mind. You've already articulated most of that. The reason we need something like GitHub, is it is SPECIFICALLY for issues - there is no guarantee developers read the user group for instance, and even if they do, there is a lot of other discussion here - and even this thread for instance, doesn't entirely focus on summarising "here is the issue" and "here is a proposed solution". GitHub is where you KNOW the developers will see it. And for instance, if they work on a solution, and come back and say "we couldn't add a setting, but we changed the hardcoded value to 4096 characters. Please try out this test version and see how it works" - you could try that version and work out whether it fixes the issue for you. I think there is a lot of "GitHub is scary because it's for developers" when in fact, you never need to venture past the "issues" section if you don't want to go near the code. Could it be better? Yes, and our updating the issue template is a way of trying to make that experience better, so I'd definitely be keen to hear your feedback once you try it.
Kind regards
Quentin