Starting from "emacs -Q -f eshell":

   ~ $ echo $exec-path[0]
   /usr/local/sbin
   ~ $ echo $exec-path[${echo 0}]
   /usr/local/sbin
   ~ $ echo $exec-path[${*echo 0}]
   ;; no output

This is because 'eshell-eval-indices' gets an S-expr describing code to 
evaluate for the indices, and it just passes that to 'eval'. That's not 
the right way to do things for Eshell: instead, we should rely on 
'eshell-do-eval', which properly handles asynchronous evaluation. That's 
required for working with external commands like "*echo" (which calls 
the real /bin/echo).

The attached patch fixes this by changing 'eshell-eval-indices' to 
'eshell-indices', which does some minimal transformations on the S-expr 
for the indices, and then uses it to build the final S-expr to pass to 
'eshell-do-eval'.

This could possibly go in Emacs 29, since it's a bugfix to add onto a 
previous bugfix (see commit 990f36fa10). However, I'd lean towards just 
merging to master; this is a fairly obscure issue, and we can't just fix 
*every* bug we find on the release branch, or the branch will never 
stabilize. If someone else thinks it's important enough to go on the 
release branch though, I won't argue.