damionmikol wrote:
Is there a reasonable explanation for why we cannot delete a signature from Apple Preview, yet all other annotations can be changed? If the date can be changed, what value is the signature?
If there is some reason for this, why was there no Save As warning?
I now have a PDF that is not usable.
Not quite sure what you are saying...
A saved signature is a bit of security risk as you might imagine. Once you plant it and save it on a document—it is not the kind of thing you would want easily removed for legal reason— on an important document or a pdf to your grandma.
You can delete signatures moving your mouse over the selection and X, yes

when adding a signature Commnad Z will undo the last commands, you can walk it back.
or select it an use the delete key
You can save and still edit, however if you close the PDF and reopen it then it is fixed.

ref: Fill out and sign a PDF form in Preview on Mac - Apple Support
"not useable" (?)
Local snapshots are for reverting to previous recent copies
Preview>File>Recent>Browse all version>Restore


ref: About Time Machine local snapshots on Mac - Apple Support