Effective differences between MAX485EESA+T and MAX485ESA+T?
The difference is the ESD protection...

All Maxim devices have a >=2kV ESD on the pins, the "E" variant is an extended ESD protection to +/-15kV. 

Source: App-Note:


  All Maxim devices incorporate ESD-protected structures on each pin to protect against electrostatic discharges encountered during handling and assembly. Maxim's standard RS-485 parts offer ≥ 2kV ESD on all their pins. Maxim's ±15kV protected parts improve upon this by offering (as one might suspect) ±15kV protection on their driver outputs and receiver inputs. Everything in life has a price, and the added ESD protection comes at a small additional cost. The ±15kV protected parts are usually the same pin out and functionality as the standard parts, allowing these parts to be easily substituted without modification to the board layout. The MAX485 and the MAX485E are examples of two such parts. The MAX485 has the standard level of protection, whereas the MAX485E has the ±15kV protection. It is important to note how our parts respond during ESD strikes. Not only will the parts not be damaged by ESD strikes less than their rated value, but they will also continue to work normally without having to recycle power. In addition, they are protected against ESD strikes while powered up, powered down, and in shutdown.


You can also see this on parametric searches, like from Mouser:




  Can it be used as drop-in replacement for the standard MAX485?


Yes, they are drop-in replacements.