Indented here-strings.

Nope, those require tabs. See first result on Google for "bash indented here string" as well as bash guide.

Yes they do. You must indent the here-string with tabs and not spaces (which sucks for people who prefer spaces). If you want to indent within the here-string, bash requires spaces and not tabs. Mixed spaces and tabs are required to get both kinds of indentation at the same time. A very common use case for this would be help text / usage strings.