no alternative keybinding for default Ctrl + j action (join lines) concat M-^

[jngbng]

M-^ may be the proper alternative keybinding that is bound to delete-indentation in Emacs.

M-^ runs the command delete-indentation (found in global-map), which
is an interactive compiled Lisp function in ‘simple.el’.

It is bound to M-^.

(delete-indentation &optional ARG)

Join this line to previous and fix up whitespace at join.
If there is a fill prefix, delete it from the beginning of this line.
With argument, join this line to following line.