TL;DR:
Minimum Specs:
- OS: Windows 10 version 21H1 (10.0.19043)
- Processor: AMD Ryzen 5 2600X, Intel Core i7-6800K
- Memory: 16 GB RAM
- Graphics: AMD Radeon RX 5700, NVIDIA GeForce 1070 Ti
- DirectX: Version 12
- Storage: 125 GB available space
- Additional Notes: SSD Required
Recommended Specs:
- OS: Windows 10/11 with updates
- Processor: AMD Ryzen 5 3600X, Intel i5-10600K
- Memory: 16 GB RAM
- Graphics: AMD Radeon RX 6800 XT, NVIDIA GeForce RTX 2080
- DirectX: Version 12
- Network: Broadband Internet connection
- Storage: 125 GB available space
- Additional Notes: SSD Required
It's probably not 100% necessary; even Ratchet & Clank: Rift Apart (which was designed to use the PS5's fast loading speeds to switch between worlds on the fly, and supports the latest DirectStorage implementation on PC) can be played off a hard drive, tests have shown. But any PC recent enough to play Starfield on really should have an SSD.
"Playable" and "good experience" don't necessarily mean the same thing though. Those rift transitions in R&C are rough on anything less than a decent NVMe SSD. Though there may be some room for improvement, as even high end NVMe drives struggle to handle these transitions as gracefully as the PS5.
Whether or not playing off an HDD is truly a dealbreaker though depends on where in gameplay the storage speed bottleneck causes problems. For Rift Apart, it's mostly just an issue with these rift transitions, and the gameplay effectively pauses while it waits for them. For an open world game it could be more problematic (i.e. pervasive traversal stutter during combat.)