Any way to maintain a partial index on only the highest element in Postgres?
The best is to take an alternative approach and not to query the large table all the time, if that is the only purpose of the index.

Create a second table that only contains a single timestamp and define a trigger on sometable that updates that small table whenever a new row arrives or an old one is updated.

This approach will only work well if you never delete the row with the latest created_at or never update created_at to an earlier timestamp.