Is it safer to upgrade Yosemite to Big Sur by upgrading to Catalina first?
The answer used to be to first upgrade to High Sierra. The reason was High Sierra included the firmware upgrade to allow a Mac to boot from APFS volumes. Also, if your Mac boots from a SSD, then the upgrade would convert the boot volume to APFS.
I can refer to this previous question: Updating to macOS Catalina gets stuck, when trying to update from OS X Mavericks. The jump to Catalina was to far, yet the current macOS Catalina - Technical Specifications state upgrading is possible from &quot;OS X 10.9 or later&quot;.
You have Yosemite and want Big Sur. The jump is the same distance. Other than that, I do not know for sure. IMO, upgrading to High Sierra first, would be safer.
You ask what changed. Well Big Sur had not be released when the question I linked to was posted.