We might be able to remove that limitation, given that it was added to MtoA six years ago.
Arnold itself will print a warning:
00:00:00 85MB WARNING | [driver_exr] output AOV "specular_indirect_skydome_light_test" has a long name, file will not be accepted by older EXR readers (36 g iven, 30 allowed)
So maybe we can forget about those older EXR readers, I will check with the team.
// Stephen Blair
// Arnold Renderer Support