Can the above message notification of back-order be customized? If so, where?
I've seen multiple posts saying
"This backorder message is comes from Item.php file
but I am unable to locate that file path within our cPanel File manager.
What could I be missing? Is there a plugin to customize this that would make things simpler?
You need to check below location to find your text message,
vendor/magento/module-catalog-inventory/Model/StockStateProvider.php at line no. 197
You can also override using en_US.csv file in your theme or module level by below line,
At theme level,
"We don't have as many ""%1"" as you requested, but we'll back order the remaining %2.","string to change your message"