This admin (Austenasia) on the MicroNations Fandom is wiping many pages with the text of 'did not improve' even though no known warning was placed on the page or message sent to the page creator. What should I do?
Since most users here are not active on that wiki, there is very little context to go on. However, according to the policy of that wiki, there are several reasons for why it could be deleted (all are speculation). You should always be able to ask the admin for the reason though, but calling them a bad admin because you don't necessarily agree with them, doesn't really help your case.
The content policy of that wiki says that articles in progress are "protected for seven days from poor quality deletion". This could mean that if you hadn't edited it in a week with the article still being in progress, it could get deleted.
The deletion policy also says that other deletion reasons could be violations of the different policies on that wiki, like the image use policy, the video policy, or the community guidelines.
You could message the admin on their wall, which you already did, but instead of blaming them of not giving any warnings (which will never end well), ask politely why the article was deleted, giving them a chance to civilly explain themselves. Admins are also just people who can make mistakes, but contacting them with "you didn't do this" or "exactly do this" are giving a very bad impression and cause them to become less inclined to respond.