Is a guest book necessary

You don't need one if you don't want it! It is absolutely not mandatory for weddings.

We end up going for two types lol (I originally wanted none).

We did a message in a bottle (beach wedding) which we plan to open maybe next year, and a guest book I got done through shutterfly (super cheap using a groupon).

I have to tell, I was so happy we did it! It was so much fun opening the guest book the morning after the wedding and reading the messages from our loved ones.

Definitely not needed, I don't think anyone would miss it. But if you want to do something simple, I recently saw a couple print a large photo from their engagement session with some white border around and had people sign that to frame for their home. There's also a lot of other simple options if you feel like doing it last minute.

It's fine. You will know who came to your wedding and that's all you need. Just got someone to sign their name in a book and then the book will just collect dust and one day someone will throw the book away. No need. You will kn ok w who cares and who came