As Nicol Bolas said, the version number corresponds directly to the version number were a symbol, function or constant was introduced.
It's probably impossible to write a working application using only the GL32, or any one namespace for that matter (maybe apart from GL11). For instance look at this snippet from my game:
this.fbo = GL30.glGenFramebuffers();
GL30.glFramebufferTexture2D(GL30.GL_FRAMEBUFFER, GL30.GL_COLOR_ATTACHMENT0, GL11.GL_TEXTURE_2D, this.fboImg, 0);
GL30.glFramebufferTexture2D(GL30.GL_FRAMEBUFFER, GL30.GL_COLOR_ATTACHMENT1, GL11.GL_TEXTURE_2D, this.fboTileCoordsImg, 0);
GL30.glFramebufferTexture2D(GL30.GL_FRAMEBUFFER, GL30.GL_DEPTH_ATTACHMENT, GL11.GL_TEXTURE_2D, this.depthImg, 0);
int status = GL30.glCheckFramebufferStatus(GL30.GL_FRAMEBUFFER);
if (status != GL30.GL_FRAMEBUFFER_COMPLETE)
throw new RuntimeException("Framebuffer creation failed with code: " + status);
IntBuffer scratchBuffer = BufferUtils.createIntBuffer(2);
I'm using glFramebufferTexture2D which came with OpenGL30, but I need the constant GL11.GL_TEXTURE_2D. Later on I need to call glDrawBuffers which comes from GL20. Nothing fishy here; I just an older function to do a newer thing, like the OpenGL docs tell me to. Besides, I don't really have a choice :)
What you should worry about are deprecated functions, like these: glBegin, glEnd, glVertex*, glNormal*, glTextCoord*, glTranslate*, glRotate*, glScale*, glLoadIdenity, glModelViewMatrix... etc. Generally everything that uses fixed pipeline or the matrix stack.
If you are worried if you are using deprecated functionality, you can force LWJGL to create a strictly core profile rendering context with an appropriate openGL version by passing an appropriate ContextAttribs to Display.create.