I think the changes pertaining to the move from indent-relative-maybe to
indent-relative-first-indent-point should be pushed independently of any
changes to the behaviour of indent-relative at BOB or its documentation.

Can they be pushed to master, and only backported to emacs-26 as
needed/applicable?

> From cefe6581558e45558907996ea39ac44e512f7944 Mon Sep 17 00:00:00 2001
> From: Alex Branham <alex.branham@gmail.com>
> Date: Thu, 14 Mar 2019 11:14:18 -0500
> Subject: [PATCH] Update documentation for indent-relative functions
>
> * lisp/indent.el (indent-relative): Document what happens when there
>   is no previous nonblank line.
> * doc/lispref/text.texi (Relative Indent): Document
>   indent-relative-first-indent-point instead of obsolete
>   indent-relative-maybe. Fix documentation of which argument from
                         ^^^
Missing double space.

> diff --git a/doc/lispref/text.texi b/doc/lispref/text.texi
> index 21c5a73f88..abcea17010 100644
> --- a/doc/lispref/text.texi
> +++ b/doc/lispref/text.texi
> @@ -2571,11 +2571,11 @@ The quick brown fox jum  @point{}ped.
>  @end example
>  @end deffn
>  
> -@deffn Command indent-relative-maybe
> +@deffn Command indent-relative-first-indent-point
>  @comment !!SourceFile indent.el
>  This command indents the current line like the previous nonblank line,
>  by calling @code{indent-relative} with @code{t} as the
> -@var{unindented-ok} argument.  The return value is unpredictable.
> +@var{first-only} argument.  The return value is unpredictable.

Just curious: is there any preference between "unpredictable" and
"undefined" for documenting return values?  If so, why?

>  If the previous nonblank line has no indent points beyond the current
>  column, this command does nothing.
> diff --git a/lisp/indent.el b/lisp/indent.el
> index 34757a43d7..25615f4113 100644
> --- a/lisp/indent.el
> +++ b/lisp/indent.el
> @@ -598,8 +598,8 @@ considered.
>  
>  If the previous nonblank line has no indent points beyond the
>  column point starts at, then `tab-to-tab-stop' is done, if both
> -FIRST-ONLY and UNINDENTED-OK are nil, otherwise nothing is done
> -in this case.
> +FIRST-ONLY and UNINDENTED-OK are nil, otherwise nothing is done.
> +If there isn't a previous nonblank line, call `tab-to-tab-stop'.

I'm not against spelling this final sentence out, but in my reading it's
already covered by the first sentence, as a non-existent line vacuously
"has no indent points beyond the column point starts at". ;)

Thanks,

-- 
Basil