This comes from http://www.classiccmp.org/pipermail/cctalk/2017-January/032069.html, where Noel Chiappa wrote:

OK, I have verified that all the tools to run and re-build MACRO-11 do in fact
run under vanilla V6.

Here is a tar file with the whole works, along with the MACRO-11 source. Just
download and explode, and read the 'README' file to start with.

It does not yet include the source for the 'bind' linker (used with the .REL
files that MACRO-11 puts out), because that's in BCPL, and there's no point in
adding the source for that until I've put the BCPL compiler itself out, which
I will do once I can confirm that it can correctly compile itself (it's
written in BCPL) and re-create itself.