Microsoft Edge allow-insecure-localhost flag removed in Version 88.0.705.50

Last week I could navigate to edge://flags/#allow-insecure-localhost and I could edit the flag so that my localhost could serve invalid SSL certificates. Today after the latest Edge update (Version 88.0.705.50) I can no longer find this flag.

Has it moved to another secret area? How can I allow insecure localhost SSL certs again for my application?

As of Edge v134, this flag is no longer available as M130 flags are now permanently expired and the #allow-insecure-localhost is set to expire in M130 as of this writing. The new way to allow insecure content is documented in Hergy's answer below

Unless Chromium re-enables the flag, Edge users will have to downgrade to v133 or below to trust localhost.

To workaround this issue on version 119-133:

1. Go to edge://flags
2. Look for the flag Temporarily unexpire M130 flags (or any lower version number there) and enable it
3. Restart the browser
4. Search for #allow-insecure-localhost again. It should appear now.

Enable it and enjoy happy developer's life.