This is addon announces in chat when you interrupt a spell. It is lightweight and fully configurable.
To bring up the configuration interface, type /ai or /announceinterrupts. You can also reach it through the interface options.
In the configuration you can choose where you want the addon to be active (raids, party instances, BGs, arenas, etc.), as well as the message to be printed when you interrupt and which channel to print it in.
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As of patch 8.2.5 announcing to custom channels is no longer supported, and if say/yell is chosen, interrupts are only printed to yourself when not in an instanced area. This is due to limitations by Blizzard.