Zend Framework – How to Override a Zend Library

Plugins only?

Yes. You can write plugins for abstract classes and plugins should always be preferred over preferences if possible.

Preferences are useful if you want to replace an implementation. I can't think of a use case to replace the implementation for all models that extend AbstractModel, if that's even logically possible. So what you probably want is to add or change functionality and this is what plugins are for.

There is no "local code pool override" mechanism anymore. Instead you can use preferences in di.xml for any class that is instantiated by the Magento object manager, i.e. used via Dependency Injection in Magento classes. This is the equivalent to a class rewrite in Magento 1.

But for classes like Zend_Date where this is not the case, this is not possible.

However you could create a composer package that loads your own Zend_Date class before the real one is loaded via autoloading:

composer.json

{
    "name": "your/custom-date",
    "autoload": {
      "files": ["src/lib/Zend/Date.php"]
    }
}

src/lib/Zend/Date.php

class Zend_Date
{
    ...
}

Edit: After you edited a composer.json file, run composer dump-autoload to update the autoloader.