The Web API deprecation policy is the same as the standard Java API :
If an API is deprecated in version X.Y, this API will be dropped in version (X+2).0. Example: an API deprecated in 4.1 is supported in 4.2, 4.3, 5.0, 5.1, 5.2, 5.3 and is dropped in version 6.0.
That does not match the description in tooltips:
A deprecated service remain available through the first L.T.S. version released after deprecation, and will be removed before the second. For example a WS deprecated in 5.2 will be deleted in a 6.X version. The same logic applies for WS parameters.