Is a form 19 legal if it is not signed?

Hi, if you're talking about Queensland and Form 19 is a Notice to vacate from mortgagee to tenants then this link explains that the form must be signed.
https://www.rta.qld.gov.au/Forms-and-publications/Forms/Forms-for-general-tenancies/Notice-to-vacate-from-mortgagee-to-tenants-Form-19

I imagine not. No point in having a place for a signature if it is not required.
Contact the authority issuing the form.
Hope this helps.