Reproduction steps:
1. Open and build the attached project for iOS.
2. Check the Xcode Console.

Actual result:
It is possible to select ETC format in texture settings for iOS, but it decompresses when the Xcode project is built.

Metal:
ETC texture format is not supported, decompressing texture.

GLES3:
GL_OES_standard_derivatives GL_KHR_texture_compression_astc_ldr GL_EXT_color_buffer_half_float GL_EXT_debug_label GL_EXT_debug_marker GL_EXT_pvrtc_sRGB GL_EXT_read_format_bgra GL_EXT_separate_shader_objects GL_EXT_shader_framebuffer_fetch GL_EXT_shader_texture_lod GL_EXT_shadow_samplers GL_EXT_texture_filter_anisotropic GL_APPLE_clip_distance GL_APPLE_color_buffer_packed_float GL_APPLE_copy_texture_levels GL_APPLE_rgb_422 GL_APPLE_texture_format_BGRA8888 GL_IMG_read_format GL_IMG_texture_compression_pvrtc

Reproduced with:
2017.3.0p4, 2018.1.0b5
iPhone7+ iOS 11.1
iPad Touch 6 iOS 10.2.1

Not reproducible with:
2017.2.1p3 (There was no ETC option in texture settings for iOS).

Note:
May be related to case 951854 .