I see that you can download a free version of Eagle as part of Autodesk’s Fusion. Also available on the company’s site are legacy versions 6.6.0 to 9.6.0 -- each of which I guess still can be ‘run as freeware’
However, I am specifically looking for version Eagle 6.3.0 – ideally for Linux and Mac, but Windows would be okay too -- viz:
AFAIK, EAGLE files can be read by several versions beyond what they were originally written with, so the 6.6.0 you've found should be able to read 6.3.0 files (and write them out in a new format.)
For some reason, the Mac versions of EAGLE are much worse at reading older-format files than the Windows version (I'm not sure about linux.) Several times when my Mac (preferred) EAGLE won't read an older file, it can be read using the same version Windows EAGLE (and written back in the newer format, so that the Mac can then read it.)