"Hide Extension" checkbox missing from Mojave Save As dialoge box

Edit:

Found a solution ;-) "Hide extension" option is in Mojave placed under pull-down in Save dialog toolbar.

Neither you, nor I can add an accessory view containing a hide extension check box to any Apple compiled application. You can send the macOS team feedback about this, though it will do little good, as they are in their own disconnected bubble.

Unfortunately, it will do no good to post a bug report or feedback to the macOS product team because they have that "not invented here" rejection mindset that justifies any change that they make, including something so keenly inane as removing a Save panel Accessory View check box and hiding it on an obscure menu that most users don't know exists.

I have an AppleScript that launches the dictation application, and then uses Speech Synthesizer to listen for an explicit command "Show Extension" that automatically invokes Finder to show the hidden extensions of selected files. When done, it quits dictation.

Unfortunately using Get info and unhide every file's extension I want is quite unproductive. It takes more time for me now! I would like to set show file extension when saving a file by default. There are rare files (if any) that I want extension to be hidden.

It is just Apple continuing to change the user interface. Chances are the Save As dialog will write the file with a hidden extension anyway, forcing you to perform a Get Info to unhide it.