So, what does this mean exactly? 2nd time a picture of mine has been removed from this website (and I notice I'm not the only one). At least the bare minimum is tell me what's wrong with the picture for it to be considered a delete-able. How am I supposed to know how to improve if nothing's told to me and it's just deleted due to "artistic" standards?

https://e621.net/wiki/show/uploading_guidelines#quality

The site being a booru rather than a gallery holds images to a minimum artist standard, if an image gets deleted for the listed reason you gave it means that it was deemed not up to the sites bar of artist quality. If you're not already I'd suggest posting to gallery sites like inkbunny, newgrounds, furaffinity, ect. As you'll have control over your posts on one of these sites and they aren't subject to site quality standards.

There was a thread almost the same as this the other day, which might clarify some things:
forum #275118

Then most people upload somebody else's art, so critique would be mostly irrelevant, but also considering how many deletions get made every day there's no way staff could give in-depth critique for each one.

If you want advice on how to improve it'd be better to upload your image to imgur, or any other image hosting site like https://pasteboard.co/, and create a forum thread asking how you can improve, as you'll probably get better responses, especially when a thread like this pops up every few days.

Quick edit:
Just with saying the staff are really busy, I thought I'd link this page of moderator actions, there's six pages already just for today:
https://e621.net/mod_action

A lot of people know they're busy, but often don't realise just how busy they actually are.

https://e621.net/forum/show/235851