JBoss EWS 5.x certification for Pega 8.8 is in-progress. Therefore we have not officially certified it for 8.7.
Note that we will be able to comment on 8.7 support once we verify it against the upcoming 8.8 release.
Most of the time the latest platform versions work without any regressions. But sometimes it breaks backward compatibility.
This is why we will need some more time - to make sure that EWS 5.x will not introduce any backward compatibility problems through 8.8 certification. If no issues are found then we can be confident of 8.7 support as well.
We have no GA date for 8.8 but this will be in the latter part of this year.