Have you considered that some people listen to text rather than read it? Do you think screen readers, non-native English speakers, or anyone else listening to these are able to reasonably differentiate between “Open Source” and “open source”?

Most readers won’t notice the difference or disregard it as meaningless and if you have to explain to everyone your position on a matter of communication it’s likely that you are the one who needs improvement with respect to your communication, not the rest of the world.