Honestly, I'm really sad to see the needs-block-near mutation specific requirement gone, I had some fun ones done for our pack :(
The block below one I don't really miss, that was confusing (Indeed I did shoot myself in the foot with that one), but please reconsider removing the block near condition.

They're still there, but as the BaseBlock Minetweaker function

True, but they are now a general growth requirement, not specific to the mutation event.

We made the design decision that 2 different requirements, one for growing and one for mutating, was a very confusing thing. So we unified them into "growth requirements".

If a base block is specified, it is still needed for mutation; its only (like you said) not specific to the mutation event anymore.