I've just been setting up a new Macbook, running MacOS Sequoia, and my previous trick I'd used to handle an ANSI keyboard in an Irish/UK English locale, specifically to remap shift-3 from "£" to "#", no longer works. So here's a replacement approach, using a Karabiner-Elements "Complex Modification" rule:
{ "description": "Change right-shift-3 to hash", "manipulators": [ { "from": { "key_code": "3", "modifiers": { "mandatory": ["right_shift"] } }, "to": [ { "key_code": "3", "modifiers": ["right_option"] } ], "type": "basic" } ] }