Link here: https://www.bluetracker.gg/wow/topic/eu-en/257369-a-further-update-to-mythic-keystones-in-905/

Still introducing an annoying decay mechanic but it isn't as bad now.

One of our goals in 9.0.5 has been to address a situation we introduced in Shadowlands with Mythic Keystones never de-leveling. We received a great deal of feedback on our planned adjustment, which is currently outlined in the 9.0.5 patch notes. With this proposed change, we would reintroduce an awkward dynamic where it wouldn’t be possible to sustain running a single Mythic Keystone dungeon per week, even if you always cleared the dungeon in time, because decay would necessitate re-leveling your keys every other week.

When we created the original logic for Mythic Keystones in 2016, we wanted to make sure that players were getting Keystones that appropriately suited their proficiency in Mythic Keystone dungeons, and we wanted to reinforce the importance of clean and well-executed dungeon runs. Having Keystones decay from one week to the next accomplished both of those goals. However, in Shadowlands, we introduced the Great Vault and more Season-driven tracking of performance in Mythic Keystone dungeons, and we unintentionally disabled the logic for de-leveling Keystones, but the reasons for the original design still apply.

We feel that the game should compensate players who consistently succeed at a certain difficulty by giving them keys that start at that level the following week, so we’re working on an update to 9.0.5 that should alleviate that concern while still satisfying the goals of the original system:

In a given week, if you finish a Mythic Keystone dungeon in time, the next week’s Great Vault will contain a Keystone of that same level. So, for example, if your highest Mythic Keystone completion during week one is a 15 that you completed in time, you will get another 15 Keystone from the Great Vault to start week two. If you run that 15 during week two and fail to meet the timer, however, and complete no other 15 (or higher) Mythic Keystone dungeons the rest of week two, you will receive a 14 Keystone from the Great Vault to start week three.

Thank you for your feedback and understanding.