There is a checkable/uncheckable option called Block offensive words under Settings → Voice input & output → Voice recognizer settings.
In Android 8.1, that setting is in Settings → System → Languages & input → Virtual keyboard → Google voice typing → Block offensive words
For Samsung phones, the settings are named slightly differently: from the Settings app, try Language & input → Voice search settings → Block offensive words. This of course only works with the integrated Google voice recognition.
For Android 4.3 (and possibly earlier versions):
Settings → Language & input → Google voice typing (settings) → Block offensive words
For Samsung S8+: Settings → General management → Language and input → On screen keyboard → Google voice typing → Block offensive words