Starting from "emacs -Q -f eshell":

   # Emacs 28
   ~ $ echo foo\
   bar

   foobar

   # Emacs 29
   ~ $ echo foo\
   bar

   foo
   bar

That is, Emacs 28 used to treat escaped newlines in the way you'd expect 
from other shells: it expands to the empty string. Now in Emacs 29, it 
inserts a literal newline.

There's also a similar bug when doing this inside double-quotes:

   # Emacs 28
   ~ $ echo "foo\
   bar"

   ("foo\\" "bar")

   # Emacs 29
   ~  $ echo "foo\
   bar"

   foo\
   bar

Here, both cases are wrong. In Emacs 29, it inserts the literal 
backslash+newline combo, but Emacs 28 is even worse: it returns a list 
of two elements! Wrapping arguments in quotes should always produce a 
string in Eshell.