System.Collections.Immutable.7.0.0\lib\netstandard2.0
Add the lines below to Packages/manifest.json
for version 7.0.0
"com.pancake.immutable": "https://github.com/pancake-llc/system-immutable.git#7.0.0",
and immutable has dependency to system-unsafe
"com.pancake.unsafe": "https://github.com/pancake-llc/system-unsafe.git#6.0.0",