I'm probably less technical than either of you, but my understanding is that it might be unsupported on SSDs for the reasons stated earlier...old-style secure erase passes are considered a waste of writes and SSD lifetime, especially when encryption is on, for the reasons expanded on below.
When encryption is on, the goal is already achieved already. What old-style secure erase passes achieved was obscuring the data previously written at those locations by overwriting them so that the next person who tried to read it would see gibberish. When good encryption is on, anyone who does not have the encryption key will see gibberish on the SSD, so the job is already done.
If the point is to "secure erase" specific memory locations even when encryption is off, that was easier with hard drives because their drive heads were much more likely to write files directly to specific memory locations as contiguously as possible. But the way SSDs work to do things like wear leveling, files are much more likely to be scattered in a million pieces distributed a lot more widely across the SSD...very difficult to recover even if unencrypted. Also, there is an abstraction layer between the OS and the controller that means where any software tells the SSD to write might not be exactly where they expect, making it difficult for a software utility to verify that you actually erased what you meant to. (Translation: A secure erase of an SSD cannot be guaranteed to be 100% effective) If the point is to secure an area when the encrypted SSD is currently unencrypted because a valid key was entered, some would probably argue that secure erase is pointless if someone already has access to the entire rest of the account.
The two links that were most informative on this are this one
https://apple.stackexchange.com/questions/146733/why-is-a-secure-erase-not-necessary-for-ssds
which led to this very informative one
https://security.stackexchange.com/questions/5662/is-it-enough-to-only-wipe-a-flash-drive-once/5665#5665
The role of the AES key is also interesting; even if you don't use FileVault it is still a key that is used with that abstraction layer, so if it's reset, nobody's going to recognize the data that used to be on that SSD anyway because the map connecting all the tiny scattered little pieces of data is gone.
After reading those I'm even more convinced that trying to secure erase an SSD is either not necessary, or not effective if attempted.
