Q: I heard about combat changes, what are the changes?
A: Blocking will no longer be with right click, it will be activated by shifting and/or walking backwards. PVP wont return to 1.8 pvp.
Also, the off-hand will no longer be able to be used for placing blocks, mining or attacking etc. It is now only for shields, compasses, maps and items of that nature but you can still hold any item or block in your off-hand.
Assuming the source is correct (do take note we don't have an official response from Jeb himself), this essentially means that they are making the off hand work like the Bedrock Edition: you won't be able to do things such as place torches, fire bows, eat food, drink potions, etc with your off hand. You can only hold things so that only items such as a map or compass would be useful. It appears they're completely removing the right click function of the off hand.
Why? Aside from actually deciding that the player shouldn't be able to do anything with their other hand, the answer appears to be (and excuse me Jeb if this is not the actual reason) that they still can't find a way to get offhand controls to work properly on Pocket Edition but they want feature parity. This is evident because they're also making shields activated using movement instead of right clicking, which would solve the shield issue on PE. But there's still no way for the touch screen controls to do 2 hands at once, so to keep feature parity they're removing the off hand's rightclick function on Java it seems.
So for which for some reason, the PE not being able to handle something needs to negatively effect every other version of the game. Java and Bedrock are not compatible with each other in the first place, so removing features to have feature parity between them because PE can't handle it doesn't make much sense.
Mojang, please just keep it the way it is on Java edition. Do whatever you want with the Bedrock version if you can't figure out the controls, but please don't mess with the Java version and drag it down because of a different verison of the game. There is no reason for this, the offhand system is fine and it's a wonderful quality of life feature, to reduce it to just using maps and such would be incredibly upsetting to a lot of players that use this feature.