This is one of my favorites because the shortened version is the actual opposite of the original. My family used the short version a lot. Hearing the long version for the first time felt kind of liberating :D
Fun fact. The jack of all trades idiom has evolved and been added to over the centuries. Here the conclusion of an analysis from stack exchange
Conclusions
To sum up, I offer this timeline of the earliest occurrences I could find for the various forms of jack of all trades and the proverbial phrases built up around it:
undefined
1618 Jack-of-all-trades
1631 Tom of all Trades
1639 John-of-all-trades
1721 Jack of all trades, and it would seem, Good at none
1732 Jack of all Trades is of no Trade
1741 Jack of all trades, and in truth, master of none
1785 a Jack of all trades, but master of none
1930 a Jack of all trades and a master of one
2007 Jack of all trades, master of none, though ofttimes better than master of one
The extra-long version of the expression may be considerably older than the 2007 earliest established occurrence might suggest—perhaps even a decade or two older. But it isn't the original form of the expression; and in comparison with the forms that arose during the 1700s, it is quite young.
Imagine "well actuallying" someone with a lie then posting it as a fact for everyone to repeat all over the internet for years. There is no direct origin and no proof that Selfridge even said it at all.
Even if Selfridge's entire existence were a collective fever dream*, the "full quote" is the better quote.
I can't imagine anyone who has worked in direct sales, at any amount of money, who genuinely believes "the customer is always right" is more correct of a saying without "in matters of taste".
That's not the full quote and before internet smart arses decided that every single idiom needed a fake "original full version" it didn't exist.
The point of the phrase is not literal though. Customer service means pleasing the customer, which means you sometimes have to act like they're right even if they're wrong.
Actually, that is a myth. "In matters of taste" was never part of the original saying. One theory was that it was coined as an alternative to the "buyer beware" mentality.
Also the 'in matters of taste' does not fit in every occasion.
If you order an expensive bottle of wine you can't return it because you don't like it. The trading ritual exists to make sure the wine doesn't have a defect. If the wine is fine but not too your taste, well then that's bad luck.