problem while adding Custom Libraries to CI3

I'm very well aware of what load_class() does ... the problem isn't in load_class(), it's that you're calling it from somewhere. And as I said:

neither CI_Loader::library(), nor any method in its stack trace call the load_class() function

Here's proof if you don't believe me:

$ git checkout 3.0.0
Note: checking out '3.0.0'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

 git checkout -b new_branch_name

HEAD is now at 3fe7949... [ci skip] Missing composer.json
$ grep 'load_class' system/core/Loader.php
load_class('Model', 'core');
$