Slack space appears when a file written to the disk does not occupy the last cluster entirely. For example, if I write a 512B file on the HDD formatted with 16KB clusters, only a single 512B sector will be written physically. The remaining 15.5KB will not be modified, potentially leaking sensitive data.
Flash memory, however, is substantially different. While write operations can work on single pages which may be smaller than cluster size, flash needs to be erased before it can be written to. Typical erase blocks are substantially larger (512KB to several MB), well above typical cluster sizes.
So, unless I use a filesystem with very large clusters, do I ever have to worry about securely erasing slack space on SSDs or SD cards?