Even if you've pledged or donated anonymously on a project, this only remains anonymous to the public. The owner of the project you've supported will always be able to see your name and certain details depending on the nature of your pledge.
If you pledged on a reward, the project owner will see your email address. If they need to post a reward to you, they will also request your postal address.
If you just made a donation but ticked the box to opt into project-related marketing, the project owner will also have your email address.
If you just make a donation and opt out of project-related marketing, the project owner will not have access to your email address.
If you selected the Gift Aid box at checkout, you will be required to leave a name and home address in order for the project owner to claim this from HMRC.
If the project you're supporting is a political campaign, you are legally obliged to leave contact details. You will not be able to pledge anonymously.
Still not sure what the difference is between a pledge and a reward? Find out here.