First of all, let's avoid "Turing-completeness" because then we might start arguing about whether a language with unrestricted recursion is or isn't Turing-complete since there are stack depth limits / memory limits / universe will end one day / etc.

I would phrase this question as "why would HTML escaping need unrestricted recursion or loops" -- since in practice config languages either have unrestricted recursion or loops (Nickel), or they don't (CUE, Dhall, Starlark).

For HTML escaping specifically, just having `.map(f)` and `.concat` available (in functional languages), or `for char in string` (in imperative languages), would be enough.

For something like HTML un-escaping, it's already trickier. If you are using recursion, your language needs to understand the concept of the string becoming "smaller" at each step. If you are using loops, `for ... in ...` is not enough anymore.

An even trickier example would be mergesort:

  merge(xs, ys) = ...

  mergeSort(xs) =
    let len   = xs.length
        left  = mergeSort(xs.slice(0, len/2))
        right = mergeSort(xs.slice(len/2, len))
    in merge(left, right)

(Not to mention that this implementation is faulty and will loop at len=1.)

Limiting stack depth at [arbitrary number] -- this is similar to (1). I don't know why configuration languages don't do it, to be honest.