When SwiftKey Flow is enabled:
It is not possible to slide left to delete a whole word, but tapping the backspace key will delete one character at a time, and a long press will delete the whole word. If you continue to hold the backspace key it will speed up.
When Gestures are enabled:
As well as the same functionality as above, you can additionally delete the last word you type by sliding your finger across the keys from right to left.
Video captions are available here.