This is related to the talent point gains based on level rather than the actual unlocking of talent points.
The method has since been restructured and now goes by the name
Disabling the talent point gains per level should be as simple as changing the if statement here from 10 to 71,
// talents base at level diff (talents = level - 9 but some can be used already)
if (level < 10)
However, removing the players ability to open the talent window until they hit level 71 will require client edits, somewhere in Interface\FrameXML\Constants.lua to be more precise, according to the interface files shared here https://github.com/tomrus88/BlizzardInterfaceCode/blob/master/Interface/FrameXML/Constants.lua#L207 (These are MOP+ interface files as they require level 15 in order to gain the first talent point.)
Creating a custom patch containing an edited version of this file is fairly simple.
Create a new MPQ archive, inside it, create a folder called FrameXML, which is located in a folder called Interface, and in it place the edited version of your Constants.lua file.