Safe to delete old "Macintosh HD - Data" partition?

It’s important to be certain that you’re deleting the correct data volume since they’re named identically:

1. In Terminal run the following command: diskutil apfs listVolumeGroups.
2. Find the volume group that contains both a System and a Data disk. That’s the one you’re going to keep.
3. The remaining volume group, which only has a Data disk, will be erased. Verify its name (“Macintosh HD - Data”). Note its device node (it will look like disk8s9 except the 8 and 9 will be different). Be sure to be VERY careful about writing this node down exactly.
4. Run the following: sudo diskutil apfs deleteVolume diskXsY, replacing the X and Y with the actual device node from Step 3. Again, be very careful with your typing here.

Yes, it is safe to delete it if you no longer want the data stored in it.