How do I create a signature (from a scanned PDF) without having to use certificate or lock document?

Seems like with every update things continue to change from the way they used to work.

The built-in features that lock the document are basically necessary mechanisms of security.

But if that is not the workflow that you're pursuing, Maybe use a customized stamp to look and appear as signature.

Be aware that, you can also use a self-signed certificate, that allows you to also customize the appearance of that digital signature.

In addition, when you edit the signature field, you can set its properties to do nothing after the signature is applied.

In other words, you can sign the document and continue to work on that form as needed.