This is the last year in which the Mint struck the Washington Quarters depicting the reverse eagle design that was used from 1932 - date. As the following year or in 1999, the Mint introduced the State Quarter program where it changed the reverse design for Washington Quarters.
The 1998-P Washington Quarter is very common and can be found in circulation even in some mint state grades. Only in MS66 and higher does it become challenging to locate. But there are enough MS66 examples that have been graded already to meet the demand. Examples in MS67 are scarce but there are probably still hundreds of examples available. In MS68, they are very difficult to find and possibly less than 100 examples exist with none known in any higher grade.