Seems questionable. The Nikon manuals do not mention heating, and normal stopping and starting recording will get around any limit.
It does mention the 4 GB file size limit (D750 page 69), and mentions the time limits from 10 to 30 minutes, which vary with 60p/30p, and with Highest/Nomal quality (D750 page 319). This is due to FAT32 file size of memory cards. But it is zero issue, just stop and restart the recording now and then.
If it was a size concern then the same limits wouldn't exist on other cameras that aren't capable of reaching even close to that size file even at the time limit.
There are many video file formats, but FAT32 4 GB file size limits do exist for memory cards. And that is about 30 minutes for 1920x1080 30p at only Normal quality.
I have a $300 Canon R300 camcorder which for file type .mp4, says 30 minutes or 4 GB, which ever is reached first. Its .mp4 is 1280x720 only.
But it also does AVCHD format (required for 1920x1080 resolution), which does not state limits except card size. It's a Blu-ray thing I think. And it will record a few hours (the battery died for me below at 1+ hour). However, it silently starts a new file about every 15 minutes, when a 1.9 GB file is reached. Here is one test recording a bit more than an hour:
Only the .mts files are necessary in an editor to put it all back together. The rest are support files for a Blu-ray format. I'm ignorant of Blu-ray, but I think you can just copy all to a Blu-ray DVD, and it includes lists to show it all together continuously. Nikon instead does Quicktime .mov format.
Also of interest is this link about a European tax on movie cameras that record longer than 30 minutes:
Why Digital Cameras Have a 30 Minute Video Recording Limit - Tested
4 GB files are definitely still an issue, but perhaps that's why DSLR don't automatically start a new file at 30 minutes?
But it is so simple to just stop and restart it. Tap a button twice. Surely the scene requires that anyway, maybe every few seconds... It is very easy to create a very boring video.