> For ~20$ I think I can buy a PDF of the standard from ISO.
You might find most standards for ~20 USD, but ISO8601 direct from iso.org will set you back 173 CHF (~200 USD) for part 1¹, 194 CHF (~220 USD) for part 2². For $20 you get only the latest amendments from them.
Meanwhile the Estonians will gladly sell you their version of part 1 for just under 30 USD.³
¹: https://www.iso.org/standard/70907.html