For dropping the い for some adjectives, I know this is a relatively common thing to do in casual speech.
Here’s an Article I Found on it, and I remember seeing a youtube short as well that I will add if I can ever find it! 
When you drop the い from an adjective like 「高い」→「高っ」、It’s usually an exclamatory phrase, i.e. you see a really expensive dress on sale in a store and go
「うわっ、高っ！」 “Wow, (that’s so) expensive!”
Similarly, 「怖っ」when being attacked is exclaiming that something is scary or causing fear, whether serious or in jest.
Some adjectives aren’t often shortened like this, like 「広い」、but It’s not uncommon to see い adjectives that don’t end in 「しい」 to have their い dropped when used as a casual exclamation.
On the other hand, ーえ ending adjectives seem similar to me, but I don’t have a concrete reason as to why it happens. It doesn’t seem like a dialect to me, but I could be incorrect on that.
I’ve noticed a lot of creators I watch will use 「すげぇ」a lot instead of 「すごい」、so if I had to bet on it I would think it’s a case of using an い adj. as an exclamatory phrase.
Edit:
A quick side-note, in regards to 「ここあんじゃん」、Online Japanese Slang and actual Japanese Speech patters are somewhat different from each other. Not only is online speech typically very casual, a number of shortcuts are often taken in terms of speaking and writing most things.
Things like 「wwww」or 「草」meaning “lol”,
「tskr」、use of exclamatory phrases, and 口癖 from personalities or game characters is common.
Watching online personalities stream/play games is a great way to immerse and hear Japanese spoken, but always remember that a lot of what you’re going to hear will be slang, casual language, and potentially offensive language in polite society.
