if you do wrap membrane around, be sure that it extends below the floor, wrapping the outside of the osb and your foundation. More than 2 inches would be ideal, even if that means digging down a little.

Again though, the lack of membrane might not impact your shed's lifespan much, just that it's "supposed to be done" due to water splashing and wicking up. Your choice of exterior sheathing looks like splashing wouldn't be an issue anyway.

if it's easy to remove the outer sheathing, you could simply wrap membrane around, ok if it's over the housewrap.

But the most important part is the pressure treated plate. Since you do have that you are most likely good to go. Especially since you have such wide eaves (which is really great, props to you for doing that), which will help keep water away from the walls so probably no problem would ever occur.

great workmanship, but some critical feedback regarding the foundation:

• why didn't you raise the floor an inch or so? If there is ever incidental settling, having a little extra height above ground will ensure it won't flood.
• it looks like the bottom 2x4 (used as a "plate") is non-pressure treated. is that true?
• You don't have proper flashing around the bottom exterior of the wall osb. something like your window membrane flashing.

Basically, I am concerned about moisture from untreated wood contact with the foundation, and how close it is to the ground. You did such a great job but it really seems you neglected to consider water damage along the foundation. I hope you live in a dry climate.

EDIT: Here is a kind of blah video, but saying the importance of pressure treated base plates: https://www.youtube.com/watch?v=2ZAvfCt6IyA Membrane flashing along the outside (under the housewrap) is also important, but a bit less than the plate.