Pledging securities with Zerodha is safe because your securities do not physically leave your demat account. As per SEBI's new pledging mechanism, when you pledge securities, they remain in your demat account. Instead, a pledge is registered in favour of the broker.
The broker must establish a dedicated demat account designated as the TMCM - Client Securities Margin Pledge Account (TMCM stands for Trading Member Clearing Member). Subsequently, the broker re-pledges these securities to the clearing corporation to obtain margins. This approach prevents brokers from misusing client securities since your securities always remain in your demat account.
To learn more, visit zerodha.com/z-connect/zerodha/announcements/understanding-the-new-margin-pledge-system.