I remember seeing couple of links that mentioned the option to do that being removed.
Probably what has happened in your situation.
There were also a couple of links that suggested using code to toggle the output. May or may not be viable in your situation. And, even if viable, some other update in the OS or audio drivers could completely undo the "fix". And those cmdlets, etc. likely took advantage of being able to have two distinct audio output paths to begin with.
For example (old thread) regarding Powershell's Set-AudioDevice
https://darkball.net/2018/03/change-audio-output-with-powershell-in-windows-10/
If you are interested in that sort of solution then google:
"select audio output powershell"
Still, again, all may be moot if the system cannot or does not simultaneously allow different audio outputs to front and rear audio ports.
Agree there is a certain appeal in having your music sent to one set of headphones via one audio port and being able to switch the output to the other audio port to use the second set of headphones. All without the need to physically unplug and plug in any headphones.
Understand that you do not have room for a switch. But there is still a simple low tech solution - one that I have used to switch between headphones and speakers.
Configure and send the audio output to the audio port of your choice. Plug in a 3.5 mm (1/8") M - F audio extension cable into the chosen audio port and place the other end of the extension cable somewhere easy to access on the desk. Then just plug in which ever set of headphones is to be used into the audio extension cable. I used a 15' (5 meter) cable just to allow extra freedom of movement.
Swapping headphones just takes a couple of seconds and avoids extra hardware, code, etc.. Just be sure that you match audio pinouts (TR, TRS, TRRS) between extension cable and headphones.