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I've not personally used it, but there's a fork that adds NSIS decompilation back in: https://github.com/myfreeer/7z-build-nsis

Why is decompiling NSIS a security concern?

In case of bugs in the decompiler. Extracting the files is still possible in newer versions, just not decompiling the installer scripts.

what is NSIS?

Nullsoft Scriptable Install System, a byproduct of Winamp that is ubiquitous in lightweight software installers for Windows.