Which libraries and platforms support ICU message format? 

ICU4C and ICU4J are the reference implementations, and most ecosystems have a library that covers at least the plural and select arguments. What each one supports:

• ICU4C (icu::MessageFormat) and ICU4J (com.ibm.icu.text.MessageFormat): the complete syntax, including selectordinal, named arguments, and number and date skeletons.
• Android: android.icu.text.MessageFormat, ICU4J's class shipped in the platform since API level 24.
• JavaScript, FormatJS (intl-messageformat, the engine behind react-intl): plural, select and selectordinal, number skeletons, a subset of the date and time skeletons, and rich-text tags.
• JavaScript, @messageformat/core (formerly published as messageformat): compiles plural, select and selectordinal messages into JavaScript functions, with CLDR plural data for all languages.
• PHP: MessageFormatter in the intl extension wraps the ICU library PHP is built against; its manual shows plural, selectordinal and ::currency/CAD skeleton examples, so what works depends on that ICU version.
• Dart and Flutter: the intl package (Intl.plural, Intl.select) and ARB files processed by Flutter's gen_l10n use the plural and select syntax, with other required and case-sensitive select matching.
• Angular: the built-in i18n supports ICU expressions in templates, {minutes, plural, =0 {just now} =1 {one minute ago} other {{{minutes}} minutes ago}} and select, and they can be nested.
• Java, java.text.MessageFormat: only partially. Its argument types are number, date, time and choice; ChoiceFormat selects by numeric range without locale plural rules, and every apostrophe starts a quote. Use ICU4J for real plurals.
• iOS and macOS: no built-in ICU MessageFormat. Plurals go through String Catalogs (Xcode 15 and later) or .stringsdict files, which store one string per CLDR category behind a %#@variable@ placeholder instead of a plural argument inside the message.

Implementations differ most in two places: whether an apostrophe starts a quote, explained in How do I escape braces and apostrophes in ICU message format?, and which skeletons they accept. Test messages in the target library, or in the ICU message editor for the reference ICU behaviour.