That message appears at the beginning of a debug session because avr-gdb was compiled without support for the Expat library, so it can’t understand some of the information received from the gdb server and issues that warning in red.
This problem was already addressed when adding support for simavr into PlatformIO and the only solution would be to recompile the avr toolchain, not an easy thing to do.
You can confirm this is the case with the GDB command show configuration
That should return something like that capture, that confirms the gdb binary was compiled with the --without-expat flag.
Right now, this warning doesn’t have any negative effect at least when simavr is used as a debug tool, because avr-gdb relies on that library to determine the flash and ram memory areas and use the respective’ of ‘vFlash’ set of packets to change memory, but if that information isn’t available then it will just consider all memory as RAM. To simavr this irrelevant because RAM and Flash are just part of the simulation and there is no special procedure required to change them, but it could have some effect when using an external debugging tool/hardware.