To operate correctly, action=purge needs to write to the database, which means it should be done using a POST rather than a GET request.
As of Gerrit change 310560,[1] action=purge will begin emitting a warning when used via GET. This should be deployed to WMF wikis with 1.28.0-wmf.20, see https://www.mediawiki.org/wiki/MediaWiki_1.28/Roadmap for the schedule.
Clients that use action=paraminfo to determine whether to use GET or POST for an action should automatically switch to POST; any others should manually switch to using POST for this action as soon as possible.
To check if your client's user agent is detected making such
submissions, you can also use ApiFeatureUsage[2] and look for
'purge-via-GET' once 1.28.0-wmf.20 is rolled out to wikis your client is using.
It is planned that this warning will be changed to an error during 1.29.
Let's avoid having a repeat of T142155,[3] update your code ASAP
instead of waiting until it breaks. Thanks.
[1]: https://gerrit.wikimedia.org/r/#/c/310560/
[2]: https://meta.wikimedia.org/wiki/Special:ApiFeatureUsage