Microsoft no longer releases new security updates for 8.1. Consequently, the "Windows Update" servers are slow, unreliable, and will never produce a fresh update rollup. This is precisely why a "Fully Updated ISO" created before January 2023 represents the final stable version of the OS. It is a time capsule of the last known good configuration. Microsoft has scrubbed most Windows 8.1 download pages from their official website. The official "Media Creation Tool" for Windows 8.1 is effectively retired; it only downloads the 2013 RTM build (version 6.3.9600.16384).
Remember: A fully updated Windows 8.1 is a finished book. It receives no new chapters. But for the hardware it was designed for, it remains a swift, stable, and surprisingly capable operating system. Just keep it off the public internet, or ensure you have a robust third-party firewall and antivirus solution. windows 8.1 fully updated iso
If you need this OS for compatibility, your best bet is to create your own ISO using the WSUS Offline method. If you must download a pre-made file, verify it against checksums provided by trustworthy sources like the "Heidoc ISO Downloader" (which unfortunately no longer supports 8.1 actively) or archived MSDN images. Microsoft no longer releases new security updates for 8