AjaxResponse::checkLastModified uses global $wgUser
In looking into callers, I found that the whole system of ajax callable functions is deprecated[1]

I propose that the system be removed.
For WMF deployed extensions, this means updating Collection and MediaWiki-extensions-FlaggedRevs to no longer use it (instead, whatever the intended replacement was)

My understanding is that, without Ajax functions, the entire AjaxResponse class can be removed, removing the use of $wgUser

Alternatively, since the function has no known callers[2] and no declared scope, I suggest it simply be removed without deprecation (i.e. breaking change)

I (DannyS712) have not assigned this to myself, since if the ajax system will be removed (option 1) its something I'm not familiar with

See also T42786 - does removing the dispatcher mean that the response can be removed as well?

[1] wgUseAjax was deprecated in 1.31 and ignored since 1.32; wgAjaxExportList was deprecated in 1.27 (T42786)
[2] Calls listed in the code search are for OutputPage::checkLastModified, which appears to be unrelated