Park hopping is still easy in 2021.
No additional reservations needed.
Our last day we did all 4 park for highlights. No issues or delays or additional reservations.
It is possible your next park is at capacity and they reserve the option to turn you away … But low attendance mid Jan 2021, we had no issues.
At the park-to-park bus, they had a CM checking your tickets for AP or hopper ticket before entering the bus, but this was not a consistant practice.