> In "emacs -Q", if the file "xyzzy" does not exist on the path, (call-process "xyzzy") now leaves this in the *Messages* buffer:
>
>   forward-sexp: Searching for program: Permission denied, xyzzy
>
> In older versions the entry in the message buffer is a true statement:
>
>   eval: Searching for program: No such file or directory, xyzzy

In the current trunk, I get "No such file on directory".

ELISP> (call-process "xyzzy")

*** Eval error ***  Searching for program: No such file or directory, xyzzy

> Repository revision: cbc507779b8f56ce0abf596416e8e3847de88e10

> Repository branch: buster

After Paul changes adding more strict file checking, there were some fixes to ease up. Perhaps you got a revision in between. I cannot say, because the repository revision of your report is likely some local change by you (it's not in the public repository).

>  (I also noticed debug-on-error is now 't' in "emacs -Q". 

C:> emacs.exe -Q --batch --eval "(princ debug-on-error)"

nil