For dictation? Haven't had Hey Siri enabled in a long time and dictation works fine. They even separated it from Siri, so even Siri itself can be disabled while dictation is enabled.
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I've had it happen here and there. Sometimes disabling and re-enabling dictation gets it going, sometimes putting the device in airplane mode and back out of it might do the trick or just restating the device, and sometimes it just comes back on its own essentially.