keymap.c

enum Layer {
	L_BASE,
	L_FN,
};

#define TAP_CTRL_BS_HOLD_CTRL 0

const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
	[L_BASE] = KEYMAP(
		..., CTL_T(KC_BSPC), ..., LT(L_FN, KC_SPC),
	),
	[L_FN] = KEYMAP(
		..., F(TAP_CTRL_BS_HOLD_CTRL), ..., KC_TRNS,
	),
};

bool is_hold(keyrecord_t *record) {
	return (record->tap.count <= 0 || record->tap.interrupted);
};

void tap_ctrl_bs_hold_mod(keyrecord_t *record, uint8_t mod) {
	if (record->event.pressed) {
		if (is_hold(record)) {
			register_mods(mod);
		}
	}
	else {
		if (is_hold(record)) {
			unregister_mods(mod);
		}
		else {
			register_mods(MOD_LCTL);
			register_code(KC_BSPC);
			unregister_code(KC_BSPC);
			unregister_mods(MOD_LCTL);
		}
	}
};

const uint16_t PROGMEM fn_actions[] = {
	[TAP_CTRL_BS_HOLD_CTRL] = ACTION_FUNCTION_TAP(TAP_CTRL_BS_HOLD_CTRL),
};

void action_function(keyrecord_t *record, uint8_t id, uint8_t opt) {
	switch(id) {
		case TAP_CTRL_BS_HOLD_CTRL:
			tap_ctrl_bs_hold_mod(record, MOD_LCTL);
			break;
		default:
			break;
	}
};