Installing an older version of Eclipse could work, and indeed it does.
I've installed Eclipse 2018-12 and the latest official (NOT prerelease) Window Builder version from the Eclipse Marketplace. Now the content assistant works fine.
IMPORTANT: You can have more than one Eclipse installation at the same time on your computer, as long as it is in a different folder. That's how I did it at least. Bad thing is, that you'll probably need another workspace separate from your everyday workspace, just for the Eclipse 2018-12 installation.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…