Adding my vote to this idea.

I would even go a bit further and suggest the feature would be:

• Unlock ALL objects on a board quickly
• Toggle Lock/Unlock all objects on a board

Here’s the use case for unlocking ALL objects on a board: How to unlock everything on a board?

This would be extremely helpful. I love using lock to make sure workshop participants (who are often unfamiliar with Miro) don’t accidentally move elements of the board around they aren't supposed to. But afterwards I want to be able to copy things (like entire frames) to a new board for synthesis, and unlocking each individual item is a huge pain. 

Yes please do add this feature!

I’m working on duplicating a board as a basis of a new board and want to delete some (but not all) of the old content and literally have to now unlock hundreds of items individually….

As per @Kate Ivanova, you can now unlock ALL objects on a board with one action!

There are two ways to do this:

1. You can go to the dropdown menu of the Locked element https://gyazo.com/d23d59c17cedf6ae12498adb274b8c34
2. Right-click on Canvas and choose the option in the dropdown menu https://gyazo.com/55967d7676bc3bb126af92f08fb8a99e