Luckily the "Customize Entry Types"-Dialog ist back! Unfortunately it is not (yet) working thoroughly:

• the "plus"-button for fields is grey and can therefore not be pressed, although I have entered a fieldname in the dropdown-box - this only changes after one clicks into the "new entry type"-box at least once
• the drag and drop to change the order isn't working at all (or at least I cannot figure out where to touch the line to drag&drop)

I have experienced this using:
JabRef 5.0--2020-03-06--2e6f433
Windows 10 10.0 amd64
Java 13.0.2

I have tested the latest development version and the problem persists:
JabRef 5.1--2020-04-23--bbea7df
Windows 10 10.0 amd64
Java 14.0.1