Discourse.ajax was actually removed back in July, and I had a deprecation notice in place that said “Discourse.ajax is deprecated. Import the module and use it instead” - generally that is the process I follow: I add backwards compatibility, output the notice to the console, and then in the next release I will remove it.
On top of that, technically speaking Discourse.ajax is a private API in the sense that it was never part of the plugin API we exposed. There’s no way for me to know the countless ways people can tap into our private APIs, so it’s hard for me to know what would be considered urgent.
Personally I don’t think it’s much to ask of plugin developers to check the consoles for their plugins once every 2-3 months and fix any problems that come up. That deprecation notice had been in stable a long time!