I think the answer to your question on retrospective banning of a development can be found here:
2.6 Spirit of the Rule
2.6.1 In case of doubt, the intention of the rule makers, which is the spirit, shall
take precedence over the letter of the rule.
2.6.2 The spirit of the rule includes, among other principles, the following considerations :
2.6.2.A Preserving general equality in overall performance between craft of different make, accepting small variations, in order to guarantee fair racing between designs of different make.
2.6.2.B Maximizing the freedom to optimize a design to personal preference and to improve the performance of a given crew and craft through refinement.
2.6.2.C The allowance to gently improve, by design, the handling and overal behaviour of a craft in small controlled steps which don't upset the balance in the class to the extent that the continued existence and growth of the class are no longer guaranteed.
2.7.3 All amendments, changes or additions shall be placed on one pre-next-season notice unless the Formula 16 authoriy considers it to be essential to act immediately to prohibit or penalize a dangerous feature.
So the answer is Yes, such a development could well be banned retrospectively.
Secretary, F16 Governing Council