If plants were not in the equation, an aquarist had a fish only tank, would a .5-1 ppm KH be a number you would promote as universally fine with all fish ( regardless of their geographic origins) as long as kept stable?
Nope, whatever cards life has dealt you (speaking geography/tap water source here) within reason
is what is likely fine for most common aquarium fish.
Discus being an exception of course liking soft acidic water, and Africans being at the opposite side of the spectrum liking hard, alkaline water. I encourage a beginner to do lots of research and ask as many questions as they feel they need to. Watching a beginner's success is super rewarding if they so choose to share their journey and ask questions / follow sound advice.
Generally speaking, 90%-95% of commonly available fish are fine in a large range of tapwater sources. Mine being very hard, never had an issue with fish before. I'm confident my fish would be perfectly safe in 0.5 degrees of kH or 15 degrees of kH (because that's exactly what they have experienced within the last 6 months with me changing from RO water back to tapwater). Absolutely 0 issues.
But, if source water is <1 degree, I would say er' on the side of boosting it to ~1 and not 0.5 degrees. This is of course speaking for most common aquaria fish species, if African Chichlids / mollies I would verge on going a little harder, or using some sort of buffer (crushed coral, Texas holy rock, limestone etc etc).
But, since this is a planted tank forum, I'll speak for myself here, I will always aim my fish keeping advice keeping plants in the cross-hairs. It's generally accepted most, if not all, plants prefer soft water, and because RO is becoming so much more popular, targeting the lowest "safe" kH range is likely the best advice for the wallet, for the ease of maintenance, and for the well being / best chance of a successful planted tank.