Yes, it was a fun time for programming languages, lots of new and ultimately seminal ideas were being explored on the pages of SIGPLAN and conference proceeding

Yes, it was a fun time for programming languages, lots of new and ultimately seminal ideas were being explored on the pages of SIGPLAN and conference proceedings. C and Pascal were practical languages and, in some programming domains, Scheme and Ada were too. If FORTRAN 77 came out in 1972 I think it might have stayed in the game, but things were developing quickly. And as you point out correctly, language features were being explored more quickly than they could be implemented. CLU even more than Alphard is an example of an academic experiment in language design that had a profound influence on the future of real programming languages. Exceptions -- C++ exception handling comes from the ideas found in CLU. Object oriented programming -- The name CLU comes from cluster because Barbra Liskov recognized the importance of compound data structures (objects, but she called them clusters) that supported an explicitly defined interface of functions (methods). These ideas were inspired by Simula (an older simulation language that had support for defining the real-world things, objects, being simulated). Immutable vs mutability -- Strings were immutable, as were other elementary types. Iterators and yield -- First introduced in CLU are now common in modern languages. Call semantics -- like lisp, data is passed by sharing a reference to the data, in contrast to the way FORTRAN or Algol passed parameters. Java and many other languages now use these call semantics, not just Lisp of Scheme. Multiple assignment -- operations (methods) could return tuples that were destructured at the point of call into multiple values. Now common in modern languages. Variable sized arrays -- as I recall, the runtime was intended to handle the autosizing of vectors. I'd never seen anything like this before 1974 (except for LISP's lists). All these important ideas and more in a language no one programmed in. Other languages were pushing in new directions too. The Algol68 Revised Report came out while I was in grad school. Wow, the language spec was so interesting that SIGPLAN made it a special edition of their monthly journal. Every concept was completely generalized: arrays could have multiple dimensions and any number of these could be left out while indexing an array value resulting in a multi-dimensional slice. I've still never seen anything like this in a modern language (is it supported by R or Mathematica? I can't remember.) Again it wasn't the implementation of Algol68 that was important (the language as specified was too hard to implement) it was the ideas it suggested to future language designers--in fact, Niklaus Wirth quit the Algol68 design committee with the idea that Algol ought to go in a simpler direction, which led to Pascal.