Partial index on OR not recommended and may work simple cases. In your case you can do this

CREATE INDEX ix1 ON bucket (x,y) WHERE y IN ["a","b","c"];
select meta().id
from bucket
where x=1 and y in ["a","b","c"]
order by y asc
offset 10 limit 100;
select meta().id
from bucket
where x=1 and y in ["a","b"]
order by y asc
offset 10 limit 100;

One easy way to verify these is create empty bucket and change query use that bucket and do EXPLAIN (no data needed for EXPLAIN)