The following patch needs to be applied when running on 1.27.x. It took us a while to run into this issue so you may not see immediate breakage after upgrading. The change is what's recommended on Manual:Messages API#Deprecated wfMsg.2A functions.
diff --git a/extensions/SphinxSearch/SphinxMWSearch.php b/extensions/SphinxSearch/SphinxMWSearch.php
index 4aea2b6..a6bed43 100644
--- a/extensions/SphinxSearch/SphinxMWSearch.php
+++ b/extensions/SphinxSearch/SphinxMWSearch.php
@@ -226,7 +226,7 @@ class SphinxMWSearch extends SearchEngine {
global $wgContLang, $wgCanonicalNamespaceNames, $wgNamespaceAliases;
// "search everything" keyword
- $allkeyword = wfMsgForContent( 'searchall' );
+ $allkeyword = wfMessage( 'searchall' )->inContentLanguage()->text();
$this->prefix_handlers[ $allkeyword ] = 'searchAllNamespaces';
$all_prefixes = array_merge(