I performed some experiments using commands to precisely teleport myself and set my pitch to 0 and yaw to 180, looking straight at the negative z axis, and determined the change in reach down to the 4th decimal place. There was no increased reach when it came to interacting with entities (attacking, right clicking, both), and the increase in block reach was almost exactly half a block (0.501 blocks). (i used a villager in a minecart to determine the reach (in a minecart to lock the xyz coords)).
Minecrafter_2020
I performed some experiments using commands to precisely teleport myself and set my pitch to 0 and yaw to 180, looking straight at the negative z axis, and determined the change in reach down to the 4th decimal place. There was no increased reach when it came to interacting with entities (attacking, right clicking, both), and the increase in block reach was almost exactly half a block (0.501 blocks). (i used a villager in a minecart to determine the reach (in a minecart to lock the xyz coords)).