How to get land_area polygons with osm2pgsql?

You can add the instruction for boundaries

node,way   boundary     text         linear

But since you have used the --hstore option, you could query it:

SELECT tags from public.planet_osm_polygon 
WHERE tags @> '"boundary"=>"administrative"'::hstore;

I asked osm2pgsql maintainers for support. Maybe this tagging should be changed. Maybe this relations shouldn't be used at all, as land areas can be constructed from boundary=administrative and coastlines (and I desparately miss a land area relation for the Netherlands).

For the meantime I use the attached query to get the polygons. Note that this has to be adjusted: It uses table name prefix "osm" instead of "planet_osm", it contains patches for some osm data errors at the time of my export, it constructs geometries in EPSG:32632 and it doesn't respect member=subarea. Additionally your planet_osm_polygon should have a hstore index; query takes under 20 seconds in my case.

WITH s1 AS (SELECT
    id rid,
    members,
    hstore(tags) tags
    FROM osm_rels
    WHERE hstore(tags) -> 'type' = 'land_area'),
s2 AS (SELECT
    s1.rid,
    m.mid,
    m.member m1,
    lead(m.member) over (partition by s1.rid order by m.mid) m2
    FROM s1, unnest(s1.members) with ordinality m(member, mid)),
s3 AS (SELECT
    rid,
    row_number() over (partition by rid order by mid) mid,
    cast(split_part(m1, 'w', 2) as bigint) wid
    FROM s2
    WHERE m2 in ('inner', 'outer') or
          m1 = 'w639948393'), -- patch data error
s4 AS (SELECT
    s3.rid,
    s3.mid,
    s3.wid,
    w.nodes
    FROM s3 LEFT JOIN osm_ways w ON w.id = s3.wid
UNION SELECT -- patch data error
    62781,
    10000 + row_number() over (order by id),
    id,
    nodes
    FROM osm_ways
    WHERE id in (742213135, 742213136)),
s5 AS (SELECT
    s4.rid, s4.mid,
    wn.wnid, wn.nid
    FROM s4, unnest(s4.nodes) with ordinality wn(nid, wnid)),
s6 AS (SELECT
    s5.rid, s5.mid, s5.wnid, s5.nid,
    st_transform(st_setsrid(st_point(
        n.lon / 10000000.0, n.lat / 10000000.0), 4326), 32632) geom
    FROM s5 LEFT JOIN osm_nodes n ON n.id = s5.nid),
s7 AS (SELECT
    rid,
    st_makeline(geom ORDER BY wnid) geom
    FROM s6
    GROUP BY rid, mid
UNION SELECT -- patch data error
    5409700,
    st_makeline(
    (SELECT DISTINCT geom FROM s6 WHERE nid = 3605382213),
    (SELECT DISTINCT geom FROM s6 WHERE nid = 4760866421)
    )),
s8 AS (SELECT
    rid,
    st_multi(st_buildarea(st_collect(geom))) geom
    FROM s7
    GROUP BY rid)
SELECT
    s1.rid,
    s1.tags,
    s8.geom
    FROM s1 LEFT JOIN s8 ON s8.rid = s1.rid
    ORDER BY s1.rid;