no-class-static-blocks

This prevents the use of Static Blocks within Classes, for example:

class Foo {
  static {
    this.x = 1;
  }
}

These will not be allowed because they are not supported in the following browsers:

What is the Fix?

You can safely use static fields, where each property is given the static keyword:

class Foo {
  static x = 1;
}

If you wish to evaluate a field using logic, you can do so with an IIFE:

class Foo {
  static x = (() => {
    if (env == "DEBUG") {
      return 4;
    }
    return Math.random();
  })();
}

This can be safely disabled if you intend to compile code with the @babel/plugin-proposal-class-static-block Babel plugin, or @babel/preset-env.