The editor used on for these in WP is not the same editor as a post or a term, or any of the other types of admin pages that ACF detects and adds fields to. To be honest, I’m not certain that ACF can even detect the admin pages or add custom field groups to them.
ACF basically adds meta boxes using the standard add_meta_box() function of WP https://developer.wordpress.org/reference/functions/add_meta_box/. If this is used on the link editor then ACF may be able to add custom fields. I don’t know how these pages work, I have not seen them used since they were disabled by default. If these pages do work in a similar way then you’d have to add custom location rules.
The next hurdle would be where “meta” for these would be stored. There is not meta table for them and ACF wouldn’t know how to store or retrieve the values. Now that I think about it, this probably means that it is impossible do to this using ACF since there isn’t an ID of any kind that you can use to tell ACF to get values associated with a link.
Since this is not really a supported feature in WP, except for backwards compatibility if the site used them in the past there’s very little chance that the developer of ACF is going to devote a lot making ACF work with them.
I would start by creating a custom post type and building the admin I want for the cpt using ACF. Then you need to figure out how you want to relate these to a post. I really don’t know how you’d do this. I think I built one site years ago that uses the links in WP.
