If you were looking for the last point where an option was set (instead of a variable), you could use the
:4verbose set is?
Last set from ~/.vim/vimrc
But currently there is no equivalent command for variables.
You could inspect the code, or use search in the source, possibly using
:vim netrw_list_hide **/* (as mentioned in the comments).
Another alternative is to use the
'verbose' option to echo everything that is executed after you set the variable. As it will be an large output echoed in the screen, you will need to save it (into a register, for example):
" in your .vimrc, after you set the variable
After the initialization ends, restore the verbose to the default value and stop the redirection:
You may need to hit Ctrl-C some times, depending on the autocmds/plugins you are using.
Then you can paste contents of the register using
"bp and analyze the "initialization log".