Issue Reinstalling MacOS on MacBook Air Late 2018

Please refer to How to reinstall macOS - Apple Support.

I know that you want to reinstall the originally installed operating system for those Macs, and that is (or was) the right way to go about selling them since the beginning of time itself. However, without any advance warning or explanation, Apple recently removed the ability to download and install "older" macOS versions through their Recovery servers. Attempting to do that will result in "the recovery server could not be contacted" screen you described.

No one at Apple seems to be interested in confirming or denying that apparent inability.

My suggestion is to refer to that Apple Support document and try one of the other startup key combinations described in Step 2 under "If you're using any other Mac".

I think we have arrived at a stopping point. If there is a silver lining to the story, it's that your Mac remains unequivocally supported and Apple can't arbitrarily dismiss your concern with the standard "your Mac is no longer supported; goodbye" response typical of other people trying to accomplish the same thing with older Macs that really aren't supported any more.

What you are doing is perfectly reasonable and there has to be a way to correctly prepare those Macs for sale — something most sellers don't bother to do. Contact Apple and ask them what you're supposed to do now: Official Apple Support

If you are fortunate enough to get someone to actually listen to you, the very first thing they are likely to do is tell you what you should have done, which is comprehensively addressed in What to do before you sell, give away, trade in, or recycle your Mac - Apple Support. Ok, so you (or whoever) didn't do that, but so what. You have a Mac that should be capable of erasing and reinstalling macOS so that it can be properly transferred to someone else.

Your next task is to write back and tell us what they had to say. Plenty of people are experiencing the same roadblock, and an answer from Apple would be nice.

They will all open macOS Recovery, so all is good so far. The only remaining question is whether each one of those startup key options will permit you to reinstall a version of macOS from Recovery for which the applicable Recovery server remains available. I don't know the answer to that question and Apple isn't talking.