I think it's probably less frequent nowadays, but it very much does happen. This still-active lawsuit[0] was made in response to LLMs generating verbatim chunks of code that they were trained on.[1]

[0] https://githubcopilotlitigation.com [1] https://www.theverge.com/2022/11/8/23446821/microsoft-openai...

You can still very trivially get entire chunks of code from Copilot including even literal author names (simply by prodding with a doxygen tag).

They do, and, early on, Microsoft (and perhaps others) put in some checks to try to hide that.

Surely they do sometimes?

ChatGPT has given me code with comments so specific I found the original 6 year old github.