Clearly, none of the maintainers found this to be a problem. They likely don't need proxy settings for themselves. And they apparently also don't want to spend the time and energy to fix an issue that they don't have. That doesn't mean it's not an issue. It's just not an issue for them and they choose to do other things in life than fix it for you. Do you really demand somebody who already gave something to you for free to do even more work for free to implement something that they don't need themselves? Why would they do it? To get you to stop complain?
It's open source, you are free to go fix it yourself. This is Hacker News, lots of people on here should have the competence to fix this. Yet, in the last 10 years, apparently nobody found it so bad that they fixed it. It is clearly not impacting as many people as you make it sound like.
"Nobody should use Deluge" is an unsuitable statement, just because it has an issue in your use case. Many people don't have that use case, and of those who do, nobody bothered to fix it yet. Complaining doesn't fix anything.
What needs just as much fixing as the raised issue is your attitude towards open source.