...even if the work notes are HTML, I doubt that would work regardless.
a list edit action is a feature from SN in the UI16 interface (perhaps it is carried through in workspace, haven't tried it yet) and I really doubt that little textbox would enable rich features like posting @ mentions (from a platform architecture perspective, as this happens while you are typing usually...not after commit).
I would STONGLY recommend against committing to modifying OOTB platform functionality like this for your business needs and instead rethink the business need.
Most of the time an ask for this is so that people can "tag" others in work that needs to be done, without having to open the ticket and wait for UI policy and client scripts to load before making a quick note like "hey @JessicaGoodWorker take a look at this".
If so, you'll want to instead approach this from a BR perspective IMO -- onChange (work_notes) if includes @ mention, then, parse the referenced user/username [[SIDE NOTE THIS WILL REQUIRE PROCESS WORK OUTSIDE OF S NOW TOO]] --> send email notification to parsed user.
** The "side note" is that users will have to CORRECTLY @ people every time...as you probably won't be able to adjust it to correct typos in the BR as it doesn't come up "live" and is done after you commit the change in the list edit...that is, unless you've got a lot of resources and/or are super ambitious ("good") with your BR scripting.
The other option you have here is to put in an idea/demand/feature enhancement request with SNow (that's not done in this community forum) to develop that rich feature for list edit actions on longform html textboxes...but I would imagine that's quite a bit of work, when Now Connect could be used for the same purpose:
IE - in Now Connect have the user instead just @ the worknote to the appropriate person, tagging the INC/SCTASK/TASK record and it should just work the same, while notifying the other employee in the Now Connect interface.
This solution wouldn't work in my organization (for example) because we use MS Teams exclusively for inter-team communication and work (not Now Connect) and that type of Organizational Change is VERY hard...but I haven't come across the ask you have here, else I'd mention that what they seek exists in Now Connect natively 😉
HOPE THIS HELPS!
