Yes, but the bulk of its code is the original TeX code, from 1984. It did not attempt to reimplement or modernise the core code. Until a few years ago, it was e

Yes, but the bulk of its code is the original TeX code, from 1984. It did not attempt to reimplement or modernise the core code. Until a few years ago, it was even still built from tex.web plus a set of change-files plus some C/C++ libraries. Nowadays, the main change-files have been merged into the WEB source, for easier management, but it's still the old TeX code at heart. Actually, there was another intermediate stage: XeTeX is in effect a descendant of TeXgX, an extended version of TeX that integrated with the now-discontinued QuickDraw GX graphics and font technology on classic Mac OS. But anyhow, it's still a direct descendant of Knuth's code. (No criticism intended: TeX was -- and still is -- a fantastic piece of work, but its code is from a different era and was shaped by constraints that are irrelevant today.)