Chrome has been going this route for a while now as it was getting so much flak for poor performance, security issues, etc. that was actually because of maliciously installed extensions. Their only option was to only allow extensions installed from their store which allows them to review the code and sign it before allowing it to be installed in the browser. All the other browsers (Firefox, Edge, etc.) are going the same/similar routes too (Firefox allows you to install from outside the store, but it has to be signed by going through a review process).

And they've been progressively making the developer mode harder to use as a backdoor for malicious extensions to install themselves, by only allowing them to run for the duration of the session. Firefox was actually the first to do this. And this is why the Extension is being 'removed' between sessions.

What is more interesting is why this particular extension was removed from the Chrome Web Store - It looks to be a fork of uBlock and lots of other Ad Blockers are still available in the store. So why is this particular one not allowed?

The only reason I know of that extensions have been removed from the store is if they violate the Single Purpose policy (https://developer.chrome.com/extensions/single_purpose) and normally only if they are doing something malicious (e.g. Claiming to be an extension that blocks ads, but actually also scraping data, injecting ads, or something like that). I know this extension is open source, but has anyone actually reviewed the code to make sure it isn't doing something malicious under the hood?

Edit: I looked at what AdNauseam does differently to normal Ad Blockers and I think where it is running afoul of the Single Purpose Policy is that it both blocks Ads AND clicks all the Ads. Now Google could be banning it from the store because it does 2 things and they should split those out into 2 separate extensions, or they are banning it because it is basically a means to commit Ad Fraud (fake clicks is a massive problem in the Ad industry). And blindly clicking every ad on a page doesn't seem like a safe thing to do personally...