This patch resolves an issue where some modifier keys could not be remapped on MacVim.

gui.c.patch

diff --git a/src/gui.c b/src/gui.c
index db59980a7..0d47396c8 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -812,7 +812,7 @@ gui_init(void)
 	    gui_mch_disable_beval_area(balloonEval);
 #endif
 
-#ifndef FEAT_GUI_MSWIN
+#if !defined(FEAT_GUI_MSWIN) && !defined(FEAT_GUI_MACVIM)
 	// In the GUI modifiers are prepended to keys.
 	// Don't do this for MS-Windows yet, it sends CTRL-K without the
 	// modifier.

This problem seems to have been mixed in with this patch:

commit f4ae6b245
Author: Bram Moolenaar <Bram@vim.org>
Date:   Sat May 30 19:52:46 2020 +0200

    patch 8.2.0851: can't distinguish <M-a> from accented "a" in the GUI

    Problem:    Can't distinguish <M-a> from accented "a" in the GUI.
    Solution:   Use another way to make mapping <C-bslash> work. (closes #6163)