Remove Macintosh HD - Data partition after downgrade to

How did you "downgrade?" You should have completely erased the entire drive which would have removed both Volumes and the Container that they were in.

If it is still remaining, you didn't "downgrade" correctly.

If you followed downgrading instructions, then you needed to reformat the drive before installing a new system or restoring an older one. I'm afraid all you can do at this point is determine on which partition your Mojave system resides and delete the other. If you are unable to do that, then you should follow these steps:

Install El Capitan or Later from Scratch

1. Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
2. Select Disk Utility from the Utility Menu and click on the Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
5. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Install OS X and click on the Continue button.

The above installs a new system, i.e., an Erase and Install. If you can restore your Mojave system from a backup, then skip the last step. Select the option to restore from a backup from the Utility Menu.

wizcreations wrote:

but first erase the drive completely and then restore from Time Machine?

Yes this is the way forward.

Internet Recovery>Disk Utilitity>View>Show All Devices>

erase/format the parent drive

then restore from your backup.

Catalina creates two partitions - Macintosh HD contains just macOS and occupies around 10-13GBs; Macintosh HD - Data contains all of the rest of your files and apps and occupies the remainder of the disk. This is quite normal. You should not delete anything. If you wish to downgrade you will have to first erase the disk which removes both partitions.