Can we circumvent the private access rules without relying on techniques that are considered “arcane” and likely to be made illegal (at some point)?

I recently read the article Profiting from the Folly of Others by Alastair Harrison, which walks through a technique to circumvent private access rules, particularly by means of (ab)using the template instantiation mechanism together with injections of inline friend definitions. I first saw this technique used in Filip Roséen’s blog article series on stateful metaprogramming back in 2015, which made use of it to implement a constexpr-counter that were, if not fully, at the very least nearly¹ standard compliant. A. Harrison’s article mentions that the origin of the technique is likely two blog articles by Johannes Schaub published in 2010-2011².