Can LeBlanc's Legend ability be used during opponent's turn? For example if they get charmed

Yes — the trigger can be used on your opponent's turn, because Deceiver is a triggered ability: triggered abilities can be put on the chain during Closed or Open States on any player's turn (Rule 383.3.c). Rule 381's "your turn only" restriction applies to activated abilities, not triggers.

The catch is the condition still has to actually happen. Hold only occurs during your own Beginning Phase, so on an opponent's turn the only possible trigger is a Conquer — and that does happen if, say, they Charm your unit into an empty battlefield: you take control there and score the conquer point even on their turn (FAQ #8299, #3426).

Two cost notes: "discard 1 and exhaust me" is a cost, so you need a card in hand and LeBlanc must be ready. Her trigger still goes on the chain while she's exhausted (FAQ #604), but you can't finalize it if you can't pay the exhaust (Rules 414.4, 422.3). And if you meant Charm-type effects hitting the legend itself, that can't happen — the Champion Legend can't be moved or controlled (Rule 107.4.d), and Charm moves units.