I use the Quickfix view in Vim often.
The text in there always has a prefix of || added to it.
So, for instance, when I copy/paste out of that buffer, etc. I get those characters included by default.
Is there a way to disable this? I haven't had luck finding any documentation or configuration for this...
It is now possible to customize the display of the quickfix window.
vim has introduced the quickfixtextfunc (:h qftf).
It allows exactly to customize the rendering of the quickfix window. The documentation includes an example, you can also see an example in the nvim-bqf README, although it's neovim/lua based.
You can see an example in the vim documentation in :h quickfix-window-function.
To implement a general-purpose qftf (not a specific one as in the vim documentation), you should start similarly than in the nvim-bqf readme, meaning, check if the info parameter quickfix field is 1, you should display items from getqflist, otherwise items from getloclist