/data/logs/modemcrash is huge and growing
This runaway modemcrash log bug was introduced by an OTA firmware update, became fairly famous, and was eventually fixed by another OTA update (I forgot the date) with a description of something like "storage optimization".

As a temporary measure, I finally made the directory read-only, so I wouldn't have to keep deleting the files; I reversed this at some point before applying the OTA update that fixed it.