ETRTO507 is a BMX tire size. Most tires are BMX tires not road tires and are wider than 40mm.
Globally, 24" size has always been a mess with at least 3 ETRTO sizes, ETRTO507, ETRTO520 and ETRTO540.
About making custom tires, that's what Brompton is doing with several Schwalbe Brompton special tires but these tires are expensive (50€/piece or more) and Brompton is producing more that 50000 bikes per year with a huge amount of existing bikes. Moreover, most of the custom ETRTO349 tires for the Brompton are derived from the same tire in ETRTO355, they are not real new tires just the same tire but 6mm smaller in diameter.