Core trac ticket: https://core.trac.wordpress.org/ticket/52961.

Thanks, @aristath, for the review.

Note: When this gets merged backported to core, object-position should simply be added in the array of allowed attributes inside the safecss_filter_attr function instead.

Right, I also create PR for it WordPress/wordpress-develop#1156.

My question is if we should keep fallback in the plugin while supporting a non backported core version.

Right, I also create PR for it WordPress/wordpress-develop#1156.

Awesome 👍

My question is if we should keep fallback in the plugin while supporting a non backported core version.

I'd say yes. The Gutenberg plugin needs to support previous versions of WP and not just the latest one, so this will need to be there.
I would suggest adding a comment in the function stating that it should be removed when the minimum required version is >= 5.8.
We could also add a check and instead of $attrs[] = 'object-position'; do

if ( ! in_array( 'object-option', $attrs ) ) {
    $attrs[] = 'object-position';
}

or instead of return $attrs; do return array_unique( $attrs );, but that may be a bit overkill, there's no real harm in having a duplicate value in that array

there's no real harm in having a duplicate value in that array

Agree. Added note about removal.

I'll merge this if it's okay with you.

Agree. Added note about removal.

Thank you :)

I'll merge this if it's okay with you.

Agreed

Changes are merged into the core. Changeset https://core.trac.wordpress.org/changeset/50649