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"
}
]
}