This is breaking the message export feature and is data loss for anyone using/relying on the output of that feature
Customer upgraded their dev server to 4.7 and are seeing the following logs whenever someone joins/leaves a channel:
Looks like part of Jonathan's fix for bad DB columns got reverted by this PR to fix the release branch after someone accidentally merged master into it https://github.com/mattermost/mattermost-server/commit/c209e4457457edc042f063390c9a222a694f3a6d
The field "Email" got re-added to the struct and table https://github.com/mattermost/mattermost-server/blame/release-4.7/model/channel_member_history.go
But our upgrade code is removing that column at the same time https://github.com/mattermost/mattermost-server/blob/release-4.7/store/sqlstore/upgrade.go#L347