Well, first, you're talking about "automatic" renaming but then describing manual renaming. Zotero does indeed rename all files you save from the web automatically based on the parent metadata, and by default it will also rename files you drag in from the filesystem as stored files. Linked files aren't renamed my default, but there's an option to do so. So generally you wouldn't need to use the manual option.

But "Rename File from Parent Metadata" certainly still exists. Can you take a screenshot of where you're looking, upload it somewhere (Google Drive, Dropbox, etc.), and provide a link here?

When I import an article or reference from the web, the downloaded pdf has the name "Full Text PDF"

No, that's just a misunderstanding. That's the attachment title, and it has always been different from the filename. The actual file is named based on the parent metadata, as you can see if you click the attachment and look in the right-hand pane or right-click → Show File.

"Rename File from Parent Metadata" appears when you right-click on the attachment, not the parent item, but again, you should almost never need to run that.

If you're actually using the Zotero 7 beta (in which case be sure to say so), there's been a change to how the manual option works — it no longer changes the attachment title to match the filename, which was never appropriate behavior. Many more details in that thread.

Since you are using the Zotero 7 beta, it's also possible you are referring to missing the right-click Manage Attachments\Rename and Move which is a feature of the Zotfile plugin (which also auto-renames PDFs as they are downloaded by the Zotero web connector).

Zotfile does not work in Zotero 7. The still-developing alternatives are discussed here:
https://forums.zotero.org/discussion/104858/on-zotfile-development-and-zotero-7#latest