deprecated
for_window [class="^.*"] border pixel 1
new_window 1pixel
thanks to deviatorslegacy's comment
Document here:
https://i3wm.org/docs/userguide.html#_default_border_style_for_new_windows
New way to do it:
default_border pixel 1
default_floating_border pixel 1
Additionally, I could assign color to border when window focus is changed. https://i3wm.org/docs/userguide.html#_changing_colors
Thanks a lot for the fix, @Antiz96 . My latest Fedora 35 update picked up 4.21 and I started seeing borders as well... this got me back to normal.