Add a toggle to enable pre 1.9 pvp. I don't know if it should be a world game rule or a personal setting but you could add a forced server side pvp mode if you need it consistent for everyone or disable forced pvp mode for personalized pvp mode.

Host Options when in-game or Game Options > Show Advanced Options > Legacy Combat/Legacy Sword Blocking. I never fully checked how the option works for servers but I don't think it works entirely on the server side (e.g. vanilla clients/L4J clients with Legacy Combat off would still have 1.9 combat)