Do active stop orders / close-only orders reserve collateral?

Stop Orders
Un-triggered stop orders do not reserve collateral - no matter if they are close-only (the default) or not. Though, they will attempt to reserve collateral (and contribute to active order holds) after they are triggered. If there is not enough collateral to cover the order at that point, the order will be rejected after triggering.

Close-Only Orders
Close-only orders do reserve collateral. Let's say a trader's maximum allowed position (given their collateral) is long or short 100 contracts and they are currently short 50 contracts. Let's also say they have existing orders to buy 150 contracts. Any new buys at that point, including close-onlys, will be rejected because those orders could not be filled along with the trader's already accepted orders.

Note: If a trader's non-close-only orders total less than their maximum position size (e.g. less than 100 in the example above), then new close-only orders will be accepted because the margin engine understands that close-onlys can only move the trader to zero. Therefore, new close-onlys are only rejected if there are active non-close-only orders (orders to open or extend a position) that would, together with the close-onlys, put a trader over their maximum position limit if filled.