Browser add-on/extension to download YouTube subscription list as an XML, in a fashion it used to work in Subscription Manager. Note: Subscription Manager's page is https://www.youtube.com/feed/channels. This add-on/extension uses ONLY that link to load, and accomplish the XML generation upon user request.
The Firefox version is live on Mozilla Add-On Gallery: https://addons.mozilla.org/en-US/firefox/addon/tube-sub-export/
For Chromium based browsers (Google Chrome, Opera, etc.) download the repo, enable developer mode in the browser's extension settings an install the extension locally. (Not a fan of the idea registering a Google Dev Account just for this, and also pay Google for that...). No harmfull code is present in the extension, you can even have a look at the actual code!
In terms of coding style, well. I know, a lot to be desired... but it works!