The filter type also changes the oil content, for example paper filters absorb much more oil than metal filters

Water chemistry makes a lot of sense. Coffee is like 99% water (a number i pulled from my ass, but its definately more than 90%).
And considering i can taste the difference between (say) san pellegrino (fuck nestle) and a more generic sparkling water, i can imagine water chemistry being important.
I know beer brewing takes it very seriously, tho perhaps beer has more delicate flavours and more long-running biochemistry.

Is it worth it for a single cup of coffee? For me, no! Because i make trash coffee.

But for someone that has spent $$$$$ on a coffee brewing setup, has roasted their own imported beans and has horrible tap water... I can understand them using distilled or RO filtered water, then customising the salts.
And I can imagine some coffee-interview talking about "their process", digging into the water chemistry thing, and it becoming more widespread amongst enthusiasts, regardless of tap watet quality