> FWIW I don't think brew has been compiling on installation even open source things by default for a while now

For built in formulas, no. For custom ones very much more so. I know I have a bunch I’ll never have bottles for and would thus always be compiled if used.

Also if you have an older version of macOS. It will try to take the compiled route for packages but also prints a stern warning that your setup is unsupported.