Has the capability for renumbering line numbers in Mach3 "Edit G Code" been removed as it used to work just fine?

Nothing has changed kin M3 for over 10 years. The gcode editor isn't part of mach, it just calls up a Winows text editor which is where your problem may be. What win version are you using?

Windows 11

If you select config/general config on the menu bar, there is a control near the top centre where the text editor is selected.  Mine is set to Notepad - what is yours?  Notepad (a windows app) doesn't have an autonumber option but there are others that do.  Notepad++ might do it.  Google is your friend.

Why do you want to number lines?