From the this makes sense file..
When you visit DNDClassics you will see this message
DMsGuild taking over for DnDClassics
With the launch of the new Dungeon Masters Guild marketplace, which combines D&D classics with 5th edition content from the community, we will soon reset all links for DnDClassics.com to redirect automatically to DMsGuild.com.
Your customer account and your library all stay with you and can be used freely at DMsGuild, DriveThruRPG, and RPGNow.
This is not the end of classic D&D titles added to our store weekly! We will continue to release more classic titles every week at DMsGuild.com and DriveThruRPG.com (until we have every last one of those gems available to you).
No timetable is set for when all will be merged so, far now you can get all your classics at DNDClassics still.