I've tested this today with the current image (image dated 2019-04-08), and it does indeed still work.
The file should be named either "ssh" or "ssh.txt", and put in the (first) partition which shows up as named "boot" - but not in a subfolder within this. If you see kernel7.img and cmdline.txt you're in the right place - it goes along-side those, and nowhere else.
Some troubleshooting tips:
- If the file is correctly parsed, it will also disappear from /boot. If it's disappearing and you still can't reach ssh, the most probable issue is that you're not finding the correct IP for it.
- Verify the card is actually booting correctly (this can take several minutes on the first boot). hdmi would be easiest. Another clue would be a distinct lack of activity from the green LED. If you load the card in your preferred partition editor, you should see almost no free space after the second partition (the partition is expanded on the first boot. If it hasn't been expanded, either the sd card is faulty or the image wasn't written correctly.)
To directly answer the question, the feature still exists as documented. If you want to verify for yourself, you can find it implemented in /lib/systemd/system/sshswitch.service.