"pirated" yes but you can often source the install images direct from Microsoft and then use open source tools to crack the activation systems. Microsoft has kind of stopped policing piracy at the individual user level, you can even download Windows cracking tools off GitHub, a platform that Microsoft owns. As long as you're using their software it makes them happy. And LTSC is worth it to have a barebones no-bullshit Windows installation.
Didn't know you could get the base images directly from Microsoft, safe third party tools to validate the installation would still concern me, unfortunately.
I'm not all that concerned with the legality of it and more about the chain of trust, if there is need to download images/tools from shady websites for the thing just to work.