It would be a rare event that a landlord or any person acting on behalf of a landlord failed to sign a section 21 notice served on a tenant but it is nonetheless a point worth considering.
In Barker v Hands  EWCA Civ 869, a landlord had served a section 21 notice which was not signed. The tenant defended proceedings on the basis that because the notice was unsigned it was invalid.
District Judge Shroder held that the notice was valid and ordered possession. The tenant appealed which was dismissed and so the tenant sought permission to appeal to the Court of Appeal. Permission to appeal was refused and the Court of Appeal said:
This second appeal raises no important point of principle or practice, and in any event the judge’s decision was clearly right.
Therefore, a section 21 notice does not need to be signed although this is never advised due to the serious delays which could be caused! Section 21 Housing Act 1988 makes no provision for a notice served under that section to be signed.
A section 8 notice is different because that is a prescribed form with a signature box so in all probability would need to be signed to be valid.