The function for user self-deleting accounts is built into release
20.09. Log into the account (web browser), and find the option under User > Preferences. If it is not there, you may have missed a point release (minor changes between full releases). Try updating.
The function for admin-deleting accounts is not new but it changed a bit in
21.01. Where that option is located in the galaxy.yml is for that release in the quote posted earlier.
These are distinct functions – maybe that is the confusion?
Note: There is a pending release undergoing testing right now:
20.05. It is live at UseGalaxy.org. The testing process usually takes a few weeks to settle out, and it is not recommended to update servers until the release is announced. It is also usually better to update in the order of releases. So, getting updated to
20.01 is recommended, to prepare for the
20.05 release/update, and to capture any intermediate bug fixes/update between releases. Get the “stable” version for most use-cases, or expect odd behavior.
*Releases — Galaxy Project 21.01 documentation
BTW – The admin docs are versioned. Use the version toggle buttons at the bottom left corner of the TOC to go to any release-specific version of the page you are viewing.