How do I restore the Option-8 bullet point shortcut in macOS 15.3.2?

What about other Option-key combinations? Is it just Option-8 that's affected?

Since Option-8 is almost universally •, and you've checked your Keyboard Input Sources for rogue languages/keyboards, the next suggestion would be an override. Two places come to mind;

First would be Settings -> Keyboard -> Text Replacements which can be used to automatically substitute words, phrases and other keystroke for other characters. See if Option-8 is listed there and replaced with something else.

The second option would be Settings -> Keyboard -> Keyboard Shortcuts where key combinations can be setup to initiate actions such as launch an app, select a menu item, etc. Also in here are overrides for the Command, Option and Control keys, so make sure they haven't been remapped (for example, it's possible to tell the OS that you want the Option key to act like the Command Key, which could result in what you're seeing).

These options aren't known to change on their own, though, so if that is the solution, either you change it (inadvertently), or someone else changed it (as a prank?).