I've been running deluge in Linux since 2019 (version 2.0+, so considerably newer than the bug report) and it appeared to have been using my SOCKS5 proxy settings correctly; I had applied some extra hardening such as disabling natpmp etc, although I'm not sure if those are required.

However I don't doubt that these kind of leaks could have happened depending on configuration / bugs etc.

Eventually I ended up changing my configuration to run deluge in a VPN container (docker) network namespace so that I could use VPN port forwarding for better speeds (supported by Mullvad).