Are there really 5+ day nonstop playthroughs? Are there just hours of no content while the streamer eats/sleeps? Why wouldn't that be split into multiple parts by the streamer, as a natural consequence of how it was recorded?
As pointed out elsewhere, past broadcasts/VODs had an autodelete horizon added years ago, so after a certain point, you'd have to reupload your content if you wanted it archived in perpetuity.
One might imagine this is just the logical followup of them adding that horizon initially, basically saying "the 1 in 200 of you who circumvented our policy, no, for real, stop that."
There have been streamers doing subathons of 30+ days. They usually eat while doing something else/watching something they will comment later, while they sleep there is either no content or some friends/moderators talk to the viewers.
So you'd hit the limit after 600 ten-minute videos, or 100 hour-long videos.
The limit also seems to apply to "Highlights" and "Uploads" but not to "Past Broadcasts", "VODs" or "Clips" for added confusion.