Garbage translation. Be careful of the New International Version. Any version based on a multiple faith confraternity is a deliberately-diluted message. In this case, the parable is nothing to do with 'flavour''.
The same passage from the 1582 Rheims New Testament, directly translated from the Latin Vulgate:
13 You are the salt of the earth.
14 But if the salt lose his virtue, wherewith shall it be salted? It is good for nothing any more but to be cast forth, and be trodden of men.
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Note the use of 'his virtue'. Wherewith' in this case means 'with what'. 'Salted' following on from 'with what' particularly means the archaic use of salted ie. giving the appearance of value by fraudulent means.
It's not about Salt . Jesus is saying there is nothing that can be added to a fundamentally-immoral man that would pass close scrutiny to be measured as True Christian Value. The man is to be tossed away.
This would fit Jesus' repeating themes of 'sorting the wheat from the chaff' and 'by their fruits you shall know them' elsewhere in the Gospels.
Don't use modern translations. Note that the King James Bible used 'savour' instead of 'virtue', an archaic use of which could mean 'reputation', so it's easy to see where the 'taste' confusion was added to the mix.
The 1750 Challoner Revision of the Rheims, deferred to the King James Translation, as it so frequently does, meaning the most-widely-available Roman-Catholic Bible is largely-Protestant.