Undocumented for-loop construct

[Klaas Vantournhout]

Dear Bash-developers,

Recently I came across a surprising undocumented bash-feature

   $ for i in 1 2 3; { echo $i; };

The usage of curly-braces instead of the well-documented do ... done
construct was a complete surprise to me and even lead me to open the
following question on stack overflow:


https://stackoverflow.com/questions/63247449/alternate-for-loop-construct

The community is unable to find any reference to this feature, except

* a brief slide in some youtube presentation by Stephen Bourne:

    https://www.youtube.com/watch?v=2kEJoWfobpA&t=2095
    Relevant part starts at 34:55

* and the actual source code of bash and the Bourne Shell V7

Questions:
1) Is there a reason why this is undocumented?
2) Can this become documented?
3) What is the historical background behind this alternative construct?

Thanks in advance,

Klaas