Will TP-Link OneMesh be coming to the Archer A9 v6 in a future firmware update?

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In the grand scheme of things, the intention is to have OneMesh for all routers and extenders. However, there is no clear cut release date of specific products. Like you referenced in the compatibility list, that is actually the best sorce to go to. The website is updated with content usually on friday. So if any new products are added, you can find out through there.