This is yet another computing mis-use of "ISO" without specifying the actual International Organization for Standardization standard number. In this case it is ISO/IEC 9995-2:2009.
Pressing ⇧ Level 2 Shift+⇮ Level 3 Shift and then releasing that chord is one of the standard ways of providing an equivalent for a ⇨ Group 2 Select key on systems like USB Human Input Devices which do not directly allow for such a key. Usually this enables access to a common second group which has a common layout that supplies a whole bunch of stuff and remains the same whilst the group 1 layout of your keyboard varies.
But this should not involve the ⎇ Alt key. Conventionally, ⇮ Level 3 Shift is ⇮ AltGr at position A08, not ⎇ Alt at position A01. You should, if the standard mechanism has been provided correctly, still be able to input the chords that you want with the ⎇ Alt key, e.g. ⇧ Level 2 Shift+⎇ Alt+C.
Where this all happens is your X keyboard map, which is what that Arch Wiki page is discussing. If ⎇ Alt is being erroneously conflated with ⇮ AltGr then your keyboard map needs fixing. Otherwise you should be able to type the chords as aforementioned.
So you have three options:
- Keep your current keyboard map and use ⇧ Level 2 Shift+⎇ Alt in chords instead of ⇧ Level 2 Shift+⇮ AltGr.
- Fix your current keyboard map so that it does not treat ⎇ Alt as ⇮ AltGr, and proceed as #1. Since your choice of window manager, dwm, is user configured by manually altering the source code and recompiling the program, compiling and applying an XKB map shouldn't be very daunting. ☺
- Switch your keyboard map, using whatever mechanism your system has for switching XKB maps, to one that is already fixed, or (less preferably, in my view) to one that does not actually have ⇨ Group 2 Select.