There was an update recently (I think 30.4.) that changed the render engine they use, since then:

Various messages like these from the game:
ShaderAPIDX8::CreateD3DTexture: Invalid color format!
Compile of "vertexlit_and_unlit_generic_ps20b" Failed:
Compile of "vertexlit_and_unlit_generic_vs20" Failed:
IDirect3DDevice9::CreatePixelShader: shaderapi's centroid mask (0x00000000) differs from mask derived from shader name (0x0000000C) for shader ps-file teeth_flashlight_ps20b ps-index 0 ps-combo 0

I dumped the shaders and did some runs

shader-db
ERROR: shaders/blackmesa/5073.shader_test failed to compile:
0:2(10): error: GLSL 1.50 is not supported. Supported versions are: 1.10, 1.20, 1.30, 1.40, 1.00 ES, 3.00 ES, 3.10 ES, and 3.20 ES

shader-db with MESA_GLSL_VERSION_OVERRIDE=150
ERROR: shaders/blackmesa/5073.shader_test failed to compile:
0:2(10): error: GLSL 1.50 is not supported. Supported versions are: 1.10, 1.20, 1.30, 1.40, 1.00 ES, 3.00 ES, 3.10 ES, and 3.20 ES

shader-db with MESA_GL_VERSION_OVERRIDE=4.5
ERROR: shaders/blackmesa/5421.shader_test failed to compile:
0:4(12): error: extension `GL_EXT_gpu_shader4' unsupported in vertex shader

if I try to advertise GL_EXT_gpu_shader4 in mesa/main/version.c (works for gpu_shader5 and others) then the OpenGL core version becomes 2.0 and the problem persists (also because I cant build 32-bit drivers on ubuntu)

   const bool ver_2_1 = (ver_2_0 &&
                         extensions->EXT_pixel_buffer_object &&
                         extensions->ARB_map_buffer_range &&
                         extensions->ARB_draw_instanced &&
                         extensions->ARB_shader_bit_encoding &&
                         extensions->ARB_gpu_shader5 &&
//                         extensions->EXT_gpu_shader4 &&
                         extensions->EXT_texture_sRGB);