Photos are in review for more than 5 months

his is my personal opinion.
There may be a problem with the picture, so I think it's a good idea to erase it quickly.
You can delete images under review. Click on Delete on the far right bottom side of the screen,

Looking at recent trends, there seem to be a few added rules.

First, pictures are not reviewed in the order they were uploaded.
First-in/first-out is a general rule which Adobe does not always follow...

Sometimes the picture posted later is reviewed first, but in that case, the picture posted earlier has a problem.
this does not mean that the bypassed images are flawed - they haven't been looked at

The picture with that problem will be rejected for review, and if it remains, the review as a whole will not proceed well.
you can't assume that

Second, I think that if you upload a problematic picture, you will be given a penalty of delaying the review time.
that is not the way it works

It would be nice to send a rejection after review, but I think not approving or rejecting the approval is giving a kind of warning with a penalty.
you do receive notification after review

Third, I would say don't question the denial of approval or the refusal to review itself.

Of course, if you know the reason, you can fix it and it will help, but there are many reasons that are difficult to understand.

I only upload pure vector illustrations, but once a year I have an inexplicable quality problem

It's better to comply. You can edit the picture and draw it again.
yes, you can always correct and resubmit any rejected images 

These days, things are reviewed in date order, but sometimes there are leftovers.
assets are not always reviewed in date order, and yes some are skipped over 

If such a defective picture is waiting for review,

I believe it will affect the entire picture, so I think it is better to delete it quickly.
no that isn't necessary. It wasn't skipped over because it was defective 

Since too many people are uploading too many pictures, it seems like everyone should upload their own pictures instead of putting a burden on Adobe.

wow