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But in both yash and zsh (when in sh emulation), the two Bourne-like shells in that list, that expansion is still subject to split+glob (and still empty removal even in zsh even when not in sh emulation), so you still need to use the awkward "${array[@]}" syntax (or "${(@)array}" or "$array[@]" in zsh which are hardly easier to type) to preserve the list (csh and tcsh have similar issues). That split+glob and empty removal is the Bourne heritage (itself to some extent caused by the Thompson shell heritage where $1 was more like macro expansion).

Or when $IFS contains multi-byte characters, or bytes not forming valid characters.

But in both yash and zsh (when in sh emulation), the two Bourne-like shells in that list, that expansion is still subject to split+glob (and still empty removal even in zsh even when not in sh emulation), so you still need to use the awkward "${array[@]}" syntax (or "${(@)array}" or "$array[@]" in zsh which are hardly easier to type¹) to preserve the list (csh and tcsh have similar issues). That split+glob and empty removal is the Bourne heritage (itself to some extent caused by the Thompson shell heritage where $1 was more like macro expansion).

Or when $IFS contains multi-byte characters, or bytes not forming valid characters.

^{¹ in yash however, "$array" behave like "${array[@]}" while zsh's "$array" behaves like "${array[*]}". Less ugly / awkward though maybe more surprising.}

In the Bourne shell, $@ and $* contained the concatenation of the positional parameters with space characters. Only $@ when quoted was special as it expanded to the same as "$*" but with the inserted spaces not quoted. You'll notice that if you remove space from $IFS, "$@" expands to just one argument in list contexts). When not quoted, $* and $@ behave like any other variable (split upon characters of $IFS, not necessarily on the original positional parameters). For instance, in the Bourne shell:

When $IFS is empty, $* are joined on space, except for $* when quoted which is joined on nothing.

In the Bourne shell, $@ and $* contained the concatenation of the positional parameters with space characters. Only $@ when quoted was special as it expanded to the same as "$*" but with the inserted spaces not quoted (with special cases for the empty list in the newer versions where it has been addressed like on Solaris). You'll notice that if you remove space from $IFS, "$@" expands to just one argument in list contexts (0 for an empty list in the fixed versions mentioned above). When not quoted, $* and $@ behave like any other variable (split upon characters of $IFS, not necessarily on the original positional parameters). For instance, in the Bourne shell:

When $IFS is empty, $* are joined on space, except for $* when quoted which is joined on with no separator.

As to the a=$@ case in your edit, that's actually one case where ksh broke compatibility with the Bourne shell.

In the Bourne shell, $@ and $* contained the concatenation of the positional parameters with space characters. Only $@ when quoted was special as it expanded to the same as "$*" but with the inserted spaces not quoted. You'll notice that if you remove space from $IFS, "$@" expands to just one argument in list contexts). When not quoted, $* and $@ behave like any other variable (split upon characters of $IFS, not necessarily on the original positional parameters). For instance, in the Bourne shell:

'set' 'a:b'   'c'
IFS=:
printf '<%s>\n' $@
printf '[%s]\n' "$@"

Would output:

<a>
<b c>
[a:b c]

Ksh88 changed that so that $@ and $* were joined with the first character of $IFS. "$@" in list context separates the positional parameters except when $IFS is empty.

When $IFS is empty, $* are joined on space, except for $* when quoted which is joined on nothing.

Examples:

$ set a b
$ IFS=:
$ a=$@ b=$* c="$@" d="$*"
$ printf '<%s>\n' "$a" "$b" "$c" "$d" $@ $* "$@" "$*"
<a:b>
<a:b>
<a:b>
<a:b>
<a>
<b>
<a>
<b>
<a>
<b>
<a:b>
$ IFS=
$ a=$@ b=$* c="$@" d="$*"
$ printf '<%s>\n' "$a" "$b" "$c" "$d" $@ $* "$@" "$*"
<a b>
<a b>
<a b>
<ab>
<a b>
<a b>
<a b>
<ab>

You'll see a lot of variations in the different Bourne/Korn-like shells including ksh93 vs ksh88. There are also some variations in cases like:

set --
cmd ''"$@"
cmd $empty"$@"

Or when $IFS contains multi-byte characters, or bytes not forming valid characters.