I looked at okio at one point but decided against it due to lack of ISO-8859-1 support. But this may not be that big of a deal to implement manually.
(Might also be good to triple-check that it's indeed ISO-8859 and not Cp-1252 - e.g. that the Cp-1252 special characters don't render correctly on Mac. Though we may still want to support reading those characters, if not writing them).
It's not really well-supported or meant for external use, and the ISO-8859-1 implementation has at least one bug (https://youtrack.jetbrains.com/issue/KTOR-3093).
I looked at okio at one point but decided against it due to lack of ISO-8859-1 support. But this may not be that big of a deal to implement manually.
(Might also be good to triple-check that it's indeed ISO-8859 and not Cp-1252 - e.g. that the Cp-1252 special characters don't render correctly on Mac. Though we may still want to support reading those characters, if not writing them).