What is the last known Crystal version that works under High Sierra?
So that I just download precompiled binary from releases which is probably my best option.
Is it possible to build the latest and greatest preferably from MacPorts (since I have already downloaded and built all toolchain and libraries needed except for Crystal itself)?
It’s hard to say, I’m afraid you’ll need to find out. I would be surprised if 1.0 doesn’t work on it, but I don’t really know for certain.
1.1.0 works indeed, and this is the last one. Everything above according to MacPorts maintainer use new library calls:
# dyld: Symbol not found: ____chkstk_darwin
# bootstrap binaries use the libpthread
# functions that were added in macOS 10.15
platforms {darwin >= 19}
Still I naively wonder does it prevent building next version from sources using working binary? Thank you!
I really can’t recall what changes we did there. But you can try and let us know 
Some libraries went EOL, so that might be a blocker.
Out of curiosity, why 10.13?