Do guarantor agreements need to be executed as a deed?

No but there are many reasons why deeds are used over a simple contracts:

1. Deeds do not need a consideration unlike a simple contract.

2. The obligations are enforceable for 12 rather than the usual 6 years years applicable under a simple contract.

3. The law may require it.

4. A deed is an instrument in solemn form and any party to it is estopped from denying the truth of any assertion he has made in it.