Hello.
Yesterday I was going to try out 8.1 on my PC so I wanted to make a fully updated Windows 8.1 ISO using NTLite, which also added .NET 4.8 to the mix.
Here it is, fully updated to 2021-05 + .NET 4.7.2 & 4.8.
https://drive.google.com/file/d/1aEW71YhWBGC4tdjZ1WXiwYxZLXvIkONt/view?usp=sharing