By default undo does work in the scratch buffer so it is something in your configuration that is keeping this from working. As a quick way to check, try running emacs -q, which skips loading your configuration, and see if you have undo in the scratch buffer there; if so, it's definitely something you have in you configuration.
You can bisect your configuration to figure out how you are deactivating undo. You can do this manually or with the help of the bug-hunter package.
Acme doesn't stand for some generic editor! It's the famous acme text editor by Rob Pike. It's an interesting editor, very different from Emacs or Vim, and yes, very mousey. In this video Russ Cox gives a great overview: https://youtu.be/dP1xVpMPn8M
I looked at the macro expansion of the form you wrote and it looks like gibberish, so I don't think the :hook keyword allows expressions to be used as hooks, you need to define a function and use the function name:
(use-package pascal ; presumably
:init
(defun remove-pascal-completions ()
(remove-hook 'completion-at-point-functions
'pascal-completions-at-point t))
:hook (pascal-mode . remove-pascal-completions))
Also, the weird single quote character you used probably doesn't work in Emacs (but maybe you have normal single quotes in your file and it's just lemmy's markdown messing things up).
I use it in all buffers whose major mode is derived from text-mode.