I'm aware that licenses w/upgrades last 6 months. Here's my question. Say a license expired the day after IPB 2.3.0 was released. That would make 2.3.0 the latest version accessible via said license, right? Obviously there were bugs in 2.3.0, fixed in later versions. Would the owner of the aforementioned license be entitled to bug fixes, or simply be stuck with a broken version of IPB?

They would be able to download 2.3.0, but not 2.3.1-onwards.
They could of course renew and get the latest version, however, (not to mention even the latest version has bugs) so I don't think "stuck with a broken version" is fair...

yup after your license expire ,you will not able to download the script from your account ,so then u need to stick with that broken softare

I'm aware later versions can't be accessed...I'm asking if there are simply bug fix patches available, rather than new features and other items released in later versions.

Nope you are stuck with what you have until you renew.

IPS typically releases patches for actual security related bugs publicly in the Company News and Updates forum. Considering software is never completely free from bugs, wouldn't you really always have 'broken' software by your logic?

Typically, updates are released via the client center, which requires updated subscriptions.

This is why it is recommended to take care when buying a used license.