Only US. Other markets barely have liquidity during daytime and get most liquidity in opening and closing auctions. Maintenance periods are actually a complication. A few more state transitions for the system, but barely used for maintenance. The only value is for upgrades, which would still be scheduled with the market down and systems up, as participants also need to transition codebases for breaking changes, a test weekend or more is required etc. These systems are extremely resilient. You most often get an incident not because the system is down but because the latency profile has changed by a few ms.