LOWER ALTITUDE IN SEVERE TURBULENCE

[LEM]

In the old (1990) B737 Training Manual there was a recommendation to descend 4000ft below optimum altitude in case of severe turbulence.

Now the 4000ft figure has been removed, it only says to descend to a lower altitude.

My questions:
1) do you know why?
2) Is it the same on other Boeing models? What's the recommendation on Airbusses?

Thank you
LEM