I've tried a reinstall and that didn't help either. And it does it on both my MacBook Pro and my Mac Studio (desktop). That tells me that it's not a computer-specific issue.
I don't know the exact reason for the crash. I only know that when the Nav Pane is open and I'm working on a long document with a lot of headings, Word begins to use up memory, and eventually crashes. I agree with Ben--recent fixes appear to have slowed the time before the crash--but it crashes reliably. If I close Nav Pane, the memory usage stops and Word never crashes.
I don't know if it's related to the size of the file, the number of headings the are used, the number of characters, etc. I'm working on a trilogy--three different files, each about 100,000 words, probably 75-100 uses of Heading 1 and Heading 2 in each document, and Word crashes with all three files. Other documents with smaller word and heading counts don't crash. (Ben: What size files are you working with before your files crash? Any pattern that you see?).
I also don't know how many other people are affected by this or how Microsoft chooses which issues to work on for its releases. All I know is that their software has this flow, it's reproducible, it's been around for a long time, it affects the use of the product in a big way, and Microsoft is aware of it. If and when they choose to fix it remains a frustrating mystery to those of us who use the product.