vim9: Needs at least version 8.2.22844

autoload/airline/highlighter.vim

@@ -13,7 +13,7 @@ let s:separators = {}
 let s:accents = {}
 let s:hl_groups = {}
 
-if !exists(":def") || (exists(":def") && get(g:, "airline_experimental", 0) == 0)
+if !exists(":def") || !airline#util#has_vim9_script()
 
   " Legacy Vimscript implementation
   function! s:gui2cui(rgb, fallback) abort