Every broker does; the question is how much and in which direction. On 185 measured fills, OANDA Japan's median slippage was 59.0 points. The fill-quality split on this page shows what share of orders filled at a better, exact or worse price than requested.

Yes — negative means price improvement: your order filled at a better price than you requested. A median at or below zero with a short positive tail is the profile you want. Positive slippage concentrated around news events suggests the broker passes volatility straight through to you.

From order history on connected accounts: each order's requested price vs its executed price, converted to symbol points. It's measured per symbol because a point means different money on EURUSD vs XAUUSD — where the broker's tick value is known, the table converts it to approximate money per 1.00 lot.