I'm using dbeaver version 5.1.3. When I use the DBMS_OUTPUT.PUT_LINE method for oracle pl/sql it's output not shown anywhere. It has a output tab. It's also blank.
Actually you can use DBMA_OUTPUT.PUT_LINE
in DBeaver (mine is version 4.3.2), maybe you were just looking at "wrong" output tab. I have attached a screenshot of my SQL editor to make my point clearer.