This appears to have moved to Edge's policies [1]. For Edge, managed by organisations, use the SSLErrorOverrideAllowedForOrigins policy [2] or, more broadly, the SSLErrorOverrideAllowed policy [3]. These can be set centrally or in the registry (see links). Setting the former policy to ["https://localhost"] works.

As of February 2025 / Edge version 133.0, the allow-insecure-localhost flag is no longer available in the temporarily unexpired flags.

[1] edge://policy

[2] https://learn.microsoft.com/en-us/DeployEdge/microsoft-edge-policies#sslerroroverrideallowedfororigins

[3] https://learn.microsoft.com/en-us/DeployEdge/microsoft-edge-policies#sslerroroverrideallowed