Where is the "secure empty trash" button in macOS Sierra?

It was gone before Sierra. See: http://www.macworld.com/article/3005796/operating-systems/how-to-replace-secure- empty-trash-in-os-x-el-capitan.html

If you are using an SSD as your Boot Drive, when your files are emptied from the Trash, they are gone, except to those willing to disassemble the parts in a clean room and use special equipment in an attempt to reconstruct part of the old data.

If you are using a Rotating drive, the blocks are added to the free list (which is really a Bitmap of contiguous available blocks), but the data Blocks are NOT erased, and any monkey with the modern equivalent of Norton Utilities might be able to bring some of them back to life.

But overwriting the data on a Rotating drive ONCE is all that is needed for non-military secrets.

There are many reasons Apple removed that feature, not the least of which it was never capable of ensuring secure data destruction with complete effectiveness, even with magnetic media. The introduction of solid state (flash) memory rendered the feature completely pointless.

There are other reasons in addition to those, none of which are unique to the Mac.

The only way to ensure data security from its creation through its destruction and all states between those two is to encrypt it. Data doesn't suddenly become sensitive just before a user decides to erase it; it's sensitive the moment it is created and ought to be treated that way for as long as it exists.

How can I delete my files securely?

Use FileVault: macOS Sierra: Encrypt the contents of your Mac with FileVault.

Don't be misled into believing some third party product is capable of securely erasing unencrypted data either. Apple retained srm in El Capitan, probably to placate users who chose to retain their illusions regarding secure data deletion for a little while longer. It too is gone in Sierra.

For more information please read Use FileVault to encrypt the startup disk on your Mac - Apple Support.

The only way to ensure data security from its creation through its destruction and all states between those two is to encrypt it. Data doesn't suddenly become sensitive just before a user decides to erase it; it's sensitive the moment it is created and ought to be treated that way for as long as it exists.

That is wonderfully eloquent.