Parameter mutablity is determined by the current state. So a previously immutable parameter can become mutable.

Terraform:

data "coder_parameter" "isimmutable" {
  name   = "isimmutable"
  type    = "bool"
  mutable = true
  default = "true"
}

data "coder_parameter" "immutable" {
  name    = "immutable"
  type    = "string"
  mutable = data.coder_parameter.isimmutable.value == "false"
  default = "Hello World"
}

Trivial Case:

1. Workspace built with isimmutable = true & immutable = "bar"
2. Workspace updates immutable = foo
   1. Breaks, because you cannot update an immutable value

Immutability source from the past

1. Workspace built with isimmutable = true & immutable = "bar"
2. Workspace updates immutable = foo & isimmutable = false
   1. This works because at validation time, mutable = true

Why ignore the previous parameter state?

Ignoring previous state is vastly simplier and more intuitive. If the previous parameter mutability affected the current build, then questions such as, "What if the last build failed? Do we take the last successful build?", etc.