I agree this needs doing. Thje old magick2vips.c was there to support vips7 compatibility -- deprecated/im_magick2vips.c has:

int 
im_magick2vips(const char *filename, IMAGE *out)
{
#ifdef HAVE_MAGICK
    /* Old behaviour was always to read all frames.
     */
    return vips__magick_read(filename, out, NULL, 0, -1);
#else
    vips_error("im_magick2vips",
        "%s", _("no libMagick support in your libvips"));

    return -1;
#endif /*HAVE_MAGICK*/
}

Which (I think) will no longer work correctly. It should maybe be HAVE_MAGICK6 || HAVE_MAGICK7, but vips__magick_read() isn't there any more anyway, so it wouldn't compile.

Now we have nip4 it's tempting to get rid of vips7 compatibility, but maybe we should keep it a bit longer.

Perhaps the simplest fix would be to change im_magick2vips to be (eg. untested!):

int 
im_magick2vips(const char *filename, IMAGE *out)
{
    VipsImage *t;

    if (!(t = vips_magickload(filename, "n", -1, NULL)))
        return -1;
    if (vips_image_write(t, out)) {
        VIPS_UNREF(t);
        return -1;
    }
    VIPS_UNREF(t);

    return 0;
}

That won't perform well, but it would let old code compile and run.

Nice catch! It looks like the registration of *magick formats in the vips7 compatibility layer also needs to be updated:

I'll open a follow-up PR for this shortly.