Thinking about it more I'm unsure now. All of the precedent I can think of revolves around reacting to a specific effect. HBT only cares that an event happened, not a specific instance of that effect.
CtM doesn't proc after Slums because it knows that the first time already occurred. By contrast, maybe HBT could fire because it knows the first time has occurred. The FAQ entry for Replacement effects also states that nothing can react to that effect specifically, but makes no broader claims about if the event is considered to have happened.
The FAQ entry on Ordinal Events also has an interesting statement that has me leaning against my initial intuition (able to use HBT, even if SS replaces):
When an ability refers to a specific ordinal instance of something happening (e.g. “the first time”, “the second time”, etc.), it refers to that instance and only that instance. If a replacement or prevent ability happens, the game still counts it toward the number of times the replaced or prevented event has occurred.
So a card is considered to have been trashed once, even if Slums replaces it. This isn't definitive, as it only specifies that the number of times is retained (not that it actually occurred).
Anyone have precedent or alternative perspective? I'm pretty sure there's precedent on this that I'm forgetting.