I have a "TP-Link Archer C6 v2 (EU/RU/JP)", as reported by "OpenWrt 21.02.0 r16279-5cc0535800 / LuCI openwrt-21.02 branch git-21.231.26241-422c175", which is working successfully.
I'm interested in reflashing this device back to stock.
I have looked at various threads on this site, but they all seem to point to unofficial downloads. For example, the following thread referenced in the wiki:
Is there a way to do a restore with a firmware downloaded from the official TP-Link website (support website, not forums)? Ideally, without having to do TFTP- that is, maybe using OpenWRT itself?
Did you also read the last message in your mentioned topic?
There is stated: Download earliest version from tp-link (unzip and rename that file).
I guess the links above in that topic are the earliest versions for this router (EU and RU). After finishing that installation you should be able to upgrade to any OEM version you still have or latest version from tp-link itself (if no steps in between are necessary, see tp-link site).