I believe when people see the information about 1.19, 1.21 - 1.23 they will ask: "Hmm... what happened to 1.20? Was this version accidentally forgotten? Was it purposely removed and for what reason?" - So having it in the list avoids this. Also having 1.18 conveys the message that this version and any prior version is obsolete.
Topic on Talk:Version lifecycle/Archive 2
Someone else said that they thought the versions needed to be cleaned up. I'm not about to get into an edit war here, but it would be good if you could subscribe to wikitech-l and be able to join in when appropriate.
Oh my g... - this was on the mailing list? This is pretty unexpected. Well, it would have been nice to have some sort of reference in the summary to this to make it easier finding out about the rationale behind it. Probably I would not have touched the page. This is however definitively not worth an edit war. Rather have people wondering about this page's content.
If you see the thread link it from this talk or an edit summary please.