Removal of unused pads?

This is not a standard practice, and should be avoided.

First: Along with providing electrical connectivity, pins also mechanically anchor a chip to the board. Each pad that's removed increases the stress on the remaining pins, which will increase the risk of the chip detaching from the board.

Second: All of the remaining pins have nothing but soldermask between them and a trace underneath them which they aren't supposed to be connected to. Soldermask is not very thick, and it's not very durable either. If the mask is breached -- from a pin vibrating against it, for instance! -- the pin may become intermittently connected to something it wasn't supposed to be.

No, it's not good practice, but if done really carefully it can open more space for routing on the top layer. This is only useful if that makes the difference in being able to drop layers and make the board cheaper. This in turn means it only makes sense for high volume products where the price of the board actually matters in the overall product, and where it is significant relative to the engineering investment.

Otherwise, there are reasons not to do this:

1. Pads are for physical mounting too, not just for making electrical connections. This matters more for some packages than others. For a 64 pin TQFP like this, removing half the pads shouldn't reduce mechanical strength to where it matters for most cases.

2. Unless you are careful to remove the pads reasonably symmetrically all around the chip, the surface tension of solder can pull the chip off center during reflow. This is a serious issue you have to think about carefully. You may also have to change the pad shapes for the remaining pads so that the center of the pull is where the chip is supposed to be.

3. You have to be really sure that the unused pins done touch anything conductive. The solder mask should take care of this, but stuff happens. If there is a lot of vibration or thermal cycling, are you sure the unused pins won't eventually rub thru the solder mask?

It would ease the layout at the expense of making the assembly more fragile. It's probably not okay according to the IPC (using solder mask as an insulator)-- see for example this thread- but I don't have specific chapter and verse to cite. Further, the soldered leads will have a relatively large gap bridged with solder because the other leads are sitting on top of solder mask, which makes the assembly even weaker.

So I think this is amateur hour stuff but it probably works okay enough and might be acceptable for a throw-away consumer product. Definitely not acceptable for a high-reliability design.