I think the crab gem was in future

Many of those were in future

Some of them were in the background and do not speak, the ones that weren't there were not included in su future because su future focuses on steven and his relationship with the gems etc. Those characters would have been included with their own episodes if su future focused on diving into different gems and their backstory i guess

I mean gems that although we saw in their corrupt form sadly we did not see them uncorrupted in the series, the movie and Future as background characters, secondary characters, etc.