Yves Laurin echoes a concern of many people:
Secure Empty Trash: We lost that option with El Capitan; how could we get it back?
I’ve seen this question come up a number of times since late in the El Capitan public beta cycle, and you can find hundreds of postings about it on Apple’s forums and elsewhere. People have a concern about this option having gone missing, even if they used it as an extra measure of security, rather than ever having had a breach due to not using it.
Secure Empty Trash was originally designed around hard disk drives (HDDs). When you perform a normal Empty Trash operation, the file isn’t immediately overwritten on your HDD; rather, the disk’s index, created as part of the formatting operation, has its entry for that file removed. The space is available for other files, but someone could still recover the data from magnetic spinning media, or at least read some of the deleted files using basic disk repair or disk analysis software.
Read more here:: How to replace El Capitan's missing Secure Empty Trash