Unfortunately, this appears to be a change in recent versions of Safari.
There’s nothing you can do in your notebook to avoid this. The change will have to come from either another Safari update, or an update to React (the library that we depend on to build the Observable interface). We’ll see if we can get some forward motion to work on a fix in the latter.
In the meantime, you can quiet the warnings by only viewing the “Logs” in your Safari console:
… although that will have the unfortunate side effect of silencing other errors as well, until you toggle back.