I was just downloading a new copy of LaTeX for my Mac and I noticed this comment on the www.latex-project.org website:
The LaTeX3 Project
LaTeX3 Project
The LaTeX3 project is a long-term research project to develop the next version of the LaTeX typesetting system.
So I looked at the link and it refers to the 1999 article by Mittelbach and Rowley describing the LaTeX3 Project.
So it's been 20 years since the Mittlebach and Rowley article came out. That's certainly long-term. Did LaTeX3 ever happen?