[Sleeep]

Frames have been improved into iframe and then deprecated in HTML 5.

Frames were a nightmare. You can't link to a page in frames, you can't bookmark it either. Frames break the back button. Come in via search engine? You're only in the main frame, your navigation frame is missing. Want to print? Lol, good luck with that. You always ended up (either intentionally or unintentionally) with a browsing session within someone else's unrelated frameset.

I also heard they were bad for screen readers.

[ChrisSD]

Nitpick: iframes haven't been deprecated in HTML5. In fact they've been extended with new attributes like sandbox and srcdoc. Loading content via javascript is often a better approach but iframes allow you to sandbox content from your website's context, e.g. to prevent XSS attacks etc.