I'm uncertain, if it makes sense to file a bug report against the xfce4-dev-tools or not. Actually upstream has got a reason to do this ;), but on Arch Linux it results in unwanted and annoying warnings :(.
$ grep egrep /usr/bin/xdt-autogen -A3 if type egrep >/dev/null 2>&1; then EGREP=egrep else EGREP="grep -E" fi
One time I reported an egrep related warning issue against the rtirq community package and upstream, but I didn't report it against e.g. AUR's ventoy-bin. However, the above if/then issue is something very special, since a workaround does cause the warnings in this particular case. The grep package does also provide a workaround. IOW two workarounds collide with each other :D.