The old version of the dev package will be removed. So you cannot compile against libraw19 anymore.
If libraw20 is not (yet) installed, it will be installed as dependency. But as there is no longer a conflict it will not remove libraw19 (which for the package system is a completely different package). In contrast, the libraw-dev is an update to the 'same' package.
Also, if someone had installed the first libraw20 versions (with or without the -dev), which had removed libraw19, after updating to the updated version without the Conflicts/Replaces nothing would prevent them from installing libraw19 again.
The only issue could be if libraw19 has a conflict against higher versions, but I strongly doubt that.
I just tested a local install by manually removing the Replaces and Conflicts lines in the libraw20 deb available on the stable ppa. I was able to install both libraw19 and libraw20 along side libraw-dev (0.20).