This fact only proves that sex roles are "continually growing" if there used to be female jockeys and there now are none. If there never used to be any significant number of female jockeys, the fact that there are none now does not show any increased sexual stereotyping.
Does the fact that there are no female jockeys show stereotyping at all? This is very hard to say. It may be that it is legitimately physically more difficult for women to be elite-level jockeys. It may be that the job needs more strength than women typically have (this is not a stereotype, this is a physical fact), but this would only limit female jockeys, not eliminate them altogether.
However, it may also be that people are socialized to think that jockeying is not a fit job for a woman. It is a very physically tough job and jockeys end up getting hurt a lot. It may be that society raises girls in such a way that they do not think of this as a potential career. It may be that society raises men in such a way that they are unwilling to hire women for this job. In these cases, it may reflect either constant or growing stereotypes.