are posix-shell-compliant continuation lines valid/supported, or not, in /etc/default/grub ?

[PGNet Dev]

a recent grub package update, in ubuntu 18LTS, is breaking /etc/default/grub by 
mangling/overwriting users' entries, in the specific case of using continuation 
lines in the file/config. and, subsequently, the upgrade process on these 
servers.

as stated clearly at

        
https://www.gnu.org/software/grub/manual/grub/html_node/Simple-configuration.html#Simple-configuration-handling

        "...
        The file /etc/default/grub controls the operation of grub-mkconfig. It 
is sourced by a shell script, and so must be valid POSIX shell input; normally, 
it will just be a sequence of ‘KEY=value’ lines, but if the value contains 
spaces or other special characters then it must be quoted
        ..."

per POSIX

        
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_02_01

        "...
        Escape Character (Backslash)

        A <backslash> that is not quoted shall preserve the literal value of 
the following character, with the exception of a <newline>. If a <newline> 
follows the <backslash>, the shell shall interpret this as line continuation. 
The <backslash> and <newline> shall be removed before splitting the input into 
tokens. Since the escaped <newline> is removed entirely from the input and is 
not replaced by any white space, it cannot serve as a token separator.

        ..."

all that's to say that continuation lines appear to be supported in 
posix-shell-compliant /etc/default/grub

they've been in use here, on 100s of servers, in grub configs for ages.

with grub 2.04 on other, non Ubu OS, it continues to work fine.

re: the use of continuation lines, ubu dev states that 

        "Regardless of what the POSIX spec says, this is highly unusual abuse 
of POSIX shell semantics, and I do not see a need to support it"

it'd be useful/helpful to get clear on what the 'official' grub project support 
for use of posix-shell-compliant continuation lines is ...

are they 'supported' as valid use in /etc/default/grub, or not? 

also useful to know/understand whether any grub update can/should mangle a 
user's /etc/default/grub.  allowed? expected?

thx!