I don't know why, but with Word Flow you could train it, teach it words, including swear words.

GBoard / Swiftkey does its darnest to ensure that it will never select a swear word and always pick 'duck' but give me an option for 'fuck'.

Flow you didn't need to be accurate, you could be 'close enough' and it figured out the word you wanted based on what you were writing. GBoard/Swiftkey get it wrong more often than I like.