Update: I would like to point out that all of those item details appear to be revisions related to the controller configuration for Portal 2. Uninstalling Portal 2 does not appear to stop the failed downloads from occurring.
Further update: Deleting everything I could find related to ID 241100 caused many more folders to spawn, one for each installed game. All claim "I/O Operation Failed" in the logs. This seems to be a very misleading error message to me, as Steam is plainly able to alter other files and I cannot find any hint of disk troubles via SMART or in the behavior of other programs. I am also able to manually create and edit files in the directories.
I suppose I'll close this as not a bug. More experimentation in the form of moving the workshop folder outside my ecrypfts-encrypted $HOME and replacing it with a symlink to the new location has resolved the issue. It appears that the encrypted name was too long for the filesystem. I have no idea how it's managed to work for years up until now. Either I missed it the entire time, or something changed with the naming scheme of at least one file, putting it over the edge.
Nothing extracted yet.
My hard drive light was constantly flashing. Investigation eventually lead me to workshop_log.txt. The following lines repeat indefinitely as long as Steam is allowed to be online, approximately once per second:
[AppID 241100] Finished Workshop download job : No Error
[AppID 241100] Starting Workshop download job (downloading )
[AppID 241100] Get details for item 2400446116 failed : I/O Operation Failed
[AppID 241100] Get details for item 2298417566 failed : I/O Operation Failed
[AppID 241100] Get details for item 1939333416 failed : I/O Operation Failed
[AppID 241100] Get details for item 1939331050 failed : I/O Operation Failed
[AppID 241100] Get details for item 1939325883 failed : I/O Operation Failed
[AppID 241100] Get details for item 1939315905 failed : I/O Operation Failed
[AppID 241100] Get details for item 1409311171 failed : I/O Operation Failed
Due to the rapid iterations, I am honestly not entirely certain if I am correct in placing those two top lines where they are; they might be meant to go after the "Get" lines.
Looking at the relevant directories (under .local/share/Steam/steamapps/workshop/content/241100/), no files appear within them. There are no known problems with the disk health or permissions. Deleting the parent directory (241100) does not change the behavior, though it did appear to delete other directories that have not since been regenerated.