I would imagine that it’s not very economically viable for them. Whilst public trends lean towards reducing their caffeine intake, this doesn’t directly translate to sales. Only 3 countries in the world have the percentage of decaffeinated coffee sales in double digits – the US is the largest, but even that is less than 20% - globally, decaffeinated currently accounts for around 9 to 10% of all coffee sales. If you combine that with the percentage of sales that coffee flavoured powders represent for Huel – then you are looking at a probably very low volume/niche flavour.
It would also reduce the profit margins. Whilst many retailers sell decaf at the same or only slightly higher price to regular – they do so at a reduced profit. The decaffeination process adds anything up to $1 USD / kilo of raw beans. That’s how it looks on paper at least – it could prove to be more popular in reality - but that’s a financial risk they would have to be prepared to take.