I’ll third the existence of this warning message after updating a variety of packages (brms, CmdStanR, CmdStan, Xcode, etc., etc. – software update bonanza!). If I recall correctly, my previous CmdStan install also threw a bevy of inconsequential warning messages on macOS.
In my case, the updates seem to have changed the the way brms interacts with CmdStanR. With silent = 1 (or 2, I assume), sampling progress is not reported. Switching to silent = 0 allows sampling progress to be reported to the console.
I think this is because of a change introduced in cmdstanr in Update handling of show_messages, add show_exceptions by andrjohns · Pull Request #746 · stan-dev/cmdstanr · GitHub and we didn’t anticipate the affect on brms’s default behavior with silent=1. I started a discussion about this with some of the other developers on the stan slack. We’ll have to decide whether we should attempt to restore the previous default behavior by updating brms to handle that change in cmdstanr or modifying what we did in cmdstanr (or neither and brms just documents this new behavior). I’m not sure yet.