To protect custom Lightning web components, the setting Use Lightning Web Security for Lightning web components is enabled by default in new Salesforce orgs. The new Lightning Web Security architecture is replacing Lightning Locker over several releases.
Where: This change applies to new Salesforce orgs that use Lightning Experience in all editions. The Lightning Web Security setting affects Lightning web components that are used in Aura-based Experience Cloud sites. It doesn’t affect LWR-based Experience Cloud sites.
Secure Components by Default with Lightning Web Security in New Orgs (salesforce.com)