In previous versions crops would grow at light level 8 and up. As of 1.14 they now only grow at light level 9 and above. This might be a feature however it was not listen in the change logs, leading me to believe it is a bug.
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For me crops don't grow at light level 8. But, when was the last Minecraft version in which crops did crow at light level 8? IIRC, it's always been at least light level 9.
In previous versions, the block above the crop needed to be light level 9 or greater, which means the crop block itself could be light level 8. Im not sure when this changed exactly, probably in a 1.14 snapshot.
After experimenting in both Java and Bedrock, I believe crops have always needed light level 9 to grow, as violine1101 stated.
I suspect that Michael Wobst's assertion was an error that resulted from standing on a crop and looking at the F3 debug screen to get the light level. However, crops only exist on farmland, and farmland is less than a full block high, so if you stand on a crop and look at the debug screen, the light level it shows you (which is measured at the bottom of your hitbox) is the level in the farmland block, which will be 1 less than the light level in the crops block assuming the dominant light source is coming from above. It's very easy to make this mistake because when we're standing on level ground, we're almost always at an integer Y coordinate, with the bottom of our hitbox in the same block as our legs. I made the same mistake for a while and only realized it when I got inconsistent results depending on whether I was standing on the crop or floating (in creative) slightly above it.
The rule cited by Nathan Ryan can then be explained as an erroneous formalization derived via logical induction from the same mistake in observation and codified (probably) on the wiki. Given that the only tool most of us have to discover the details of Minecraft's game mechanics is by generalizing from experiments, such mostly-but-not-quite-correct rules are pretty common there.
So the only remaining question is whether crops ever grew in light level 8, as Nicolas Jacobs (the author of this report) asserted. Since there was no mention of it being changed in any changelog, and the F3 screen theory can explain why it might appear that the light level was 8 when it was really 9, I suspect it has not been changed, that crop growth requires light level 9 and always has.
Disclaimer: I'm mainly a Bedrock mod, so Java players might justifiably doubt my conclusions. I got involved because we have an old Bedrock bug report (MCPE-36863) claiming that block updates don't cause insufficiently lit crops to uproot as they do in Java. That's true, but I think it has always been true in Bedrock, so it doesn't qualify as a parity error. However, to make sure I understood the whole situation I needed to do a complete survey of the game mechanics of both editions with respect to farmland lighting and hydration, and that led me to this bug report (as well as to MC-132491).
Cannot reproduce. Crops grow for me at light level 8+.