With the merge of Gerrit change 324224,[1] action=clearhasmsg will return an error if used via GET rather than POST. The change should be deployed to WMF wikis with 1.29.0-wmf.5, see https://www.mediawiki.org/ wiki/MediaWiki_1.29/Roadmap for the schedule.
This should have very low impact: analysis of logs indicates that only Huggle[2] is using it, and Huggle has already been updated.
[1]: https://gerrit.wikimedia.org/r/#/c/324224/ [2]: https://en.wikipedia.org/wiki/Wikipedia:Huggle