Note that is SECURE erase that is not available. You can erase/reformat an encrypted SSD, I did it just a few days ago but that was a simple erase that only erased the volume directory. The content of the drive remained as encrypted text which will be overwritten as the drive is used.

NOTES:
  1. Be forewarned: Filevault encrypts every sector on the drive, whether there is data in that sector or not, so it can take a long time — even days — to complete. You can continue to use the drive normally while encryption is going on but you will likely experience a slowdown in performance while it is happening.
  2. You can format a drive APFS (encrypted) or HFS+ (encrypted) and that will work if it is a data only volume.
  3. You cannot install MacOS on an empty volume formatted APFS (encrypted) or HFS+ (encrypted). You must make the initial installation on an unencrypted volume and THEN turn on Filevault to encrypt the volume
  4. You can upGRADE or upDATE an existing copy of MacOS on an encrypted volume.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein