OpenGL
No
The v1.20.8 OpenGL GLSL spec says "No". Section 3.1 "Basis. Character Set" contains:
There is no line continuation character.
See: http://www.opengl.org/registry/doc/GLSLangSpec.Full.1.20.8.pdf or http://www.opengl.org/registry/doc/GLSLangSpec.1.50.09.pdf
Well, yes
In a later version (v4.30.6) of the spec support for line-continuation characters was added. See: http://www.opengl.org/registry/doc/GLSLangSpec.4.30.6.pdf. I'm not clear when this was added.
Older "#version" directives like (#version 100
) do not seem to disable line continuations (at least on my Nvidia desktop driver).
OpenGL ES
No, maybe
Section 1.5 "Compatibility" says:
Support of line continuation and support of UTF-8 characters within comments is optional in GLSL ES 1.00 when used with the OpenGL ES 2.0 API. However, support is mandated for both of these when a GLSL ES 1.00 shader is used with the OpenGL ES 3.0 API.
See http://www.khronos.org/registry/gles/specs/3.0/GLSL_ES_Specification_3.00.3.pdf
Summary
Newer GLSL specifications include support for line continuation characters, but in practice this support is not disabled for explicitly versioned shaders. Older GLSL compilers may not support line continuation characters, so for maximum compatibility they should be avoided.