Good point. I have just released 2.0.0.
A bit late to change the actual versions.cfg of the release.
The versions.cfg of Plone 5.1.0 already pins ZODB to 5.3.0.
So in order to use an older ZODB on Plone 5.1, you would already have had to downgrade this ZODB pin manually. I would expect problems running buildout then, but if someone has actually done this, then with Plone 5.1.5 they would simply have one extra package to downgrade (
plone.recipe.zeoserver down to 1.4.2.).
So seems okay.