As part of work on T198935[1], the representation of preferences `email-blacklist` and `echo-notifications-blacklist` returned by API meta=userinfo has changed. Previously, they were by mistake returned as arrays, which is unheard of in the API and resulted in weird output, especially in XML:

<email-blacklist> <_v>123</_v> <_v>456</_v> </email-blacklist>

At the same time, to change these preferences, you still had to use the same format as DB storage, strings with newline-separated numbers. To address this as well as a few other problems with conversion of preferences on the storage-HtmlForm boundary, a fix[2] has been implemented that turned these preferences newline-separated strings everywhere.

This fix will be deployed to WMF production next week. According to our API usage analysis, there are unlikely any users this might affect, however still please make sure you're ready.

---- [1] https://phabricator.wikimedia.org/T198935 [2] https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/444273/