What's the difference between the quarkus group-id io.quarkus.platform and io.quarkus?
In older versions (<2.0.0) the latter (without platform) was used. However, in the new sample projects the group-id io.quarkus.platform is used.
The quarkus team is reorganizing the dependencies to create a more modular and maintainable project, with common dependencies organized in platform groups, here you can watch a video where they explain everything. As part of the on going changes they reorganized the boms so this is way you see the platform suffix.
UPDATE: as @geoand pointed in this link you can find a complete article about the ongoing changes regarding the quarkus bom and platforms.