I agree being able to edit S3 tags and other metadata would be great.

Thanks for your suggestion.

I have added this request to the tracker:
Issue 1968 – Display/Modify S3 metadata like tags and storage class
You can vote for it there.

I work with files on S3 extensively, using WinSCP for almost everything, except when checking or editing S3 tags, for which I have to use the AWS management console. It's probably asking a lot, but would be very helpful if WinSCP could at least display tags. If could edit and create, even better.
Being able to display s3 storage class also nice, though check this much less often.