So the Illusioner mob has subtitles within the default resource pack, but the game doesn't show them. Minecraft doesn't show the subtitles for the Drowned mob either, however I haven't checked to see if they're in the default resource pack.
Environment
Operating system: macOS 10.13.4 Beta (build 17E182a)
Java version: 10.0.1
dupes MC-91893