Similarly you can opt your individual account out on the ChatGPT side. [0] Although by default they do seem to use both vanilla and Plus conversations for training.
Are you sure about that? Parent was not referring to the "APP -> Data controls -> Chat History & Training" Radio Button which would delete your history older than 30 days, but rather to a specific form you can use to opt out.
Coincidentally I just used this form yesterday and got the confirmation about opting out.
Edit: This was previously a comment where I said I thought it did no longer show history. As it turns out the website version of chatGPT is just having a ton of issues. So it is very slow to load and when it does finally seems to load the sidebar doesn't show any content. After refreshing a few times the sidebar does show up.
So if people are going through the steps now they might indeed think that they no longer have access to advanced features.
[0] https://privacy.openai.com/policies