I was able to work around the greyed-out Unlink option, as follows:

1. Copy and paste the INDD-based text into a spare frame.
2. Link the problematic frame that was linked to a since-deleted Word file to a spare .txt file instead (I used an Instructions.txt file.)
3. In Links, right-click on the .txt file item and click the now-available Unlink option.
4. Delete any text in the story and paste the actual text from the spare frame back in.

This is obviously a cumbersome fix. I have limited experience with placing text linked to a Word file, so I don’t know if the Unlink option being greyed out is common. What brought me to this thread via web search was a packaged document that included a now-unnecessary Word file that I deleted from the Links folder before opening the INDD file to poke around. In the future, I’ll just try to unlink before deleting.