How do you securely erase an SSD ?

As SSD (the whole drive) can be securely erased, that’s always been true. The drive is completed overwritten/filled with random data and/or zeros multiple times, the original data are obliterated for good.

I suspect the discussion you’re thinking of was around securely erasing individual files on the drive, which can be done with an HDD but not with an SSD, and that’s why Apple removed the Secure Empty Trash option.

If you are running macOS 10.12.6+, then you can boot into Recovery Mode (Command + R) or Internet Recovery Mode (Command + Option + R) in order to use Disk Utility to erase the SSD and reinstall macOS. You can also create a bootable macOS USB installer.

Since you have Filevault enabled any simple erase procedure will destroy the Filevault encryption key making it impossible for anyone to decrypt the data on the drive. Usually if an SSD has TRIM enabled, then a simple erase using Disk Utility should also be enough to cause the SSD to automatically clear out the unused NAND cells on the SSD. There is no need to write zeroes or random data to an SSD as that will cause unnecessary wear on the SSD and it may not access all of the NAND cells on the SSD. If an SSD doesn't have TRIM or Filevault enabled, then it would be best to enable one or the other before erasing the drive with Disk Utility.

Here is what Apple recommends you do when selling or recycling a Mac:

https://support.apple.com/en-us/HT201065

Put it in an external enclosure and Erase it with Disk Utility, click Security Options and pick one of the overwrite options.