Why has this hook been removed? It's such a shame... Configuring things in globals seems very dirty to me.
Topic on Extension talk:PluggableAuth
Per Extension:PluggableAuth#Group_Synchronization, "In version 7.0.0 and later when you are using an authentication plugin that supports retrieval of attributes from the identity provider (currently OpenID Connect, SimpleSAMLphp, WSOAuth, and JWTAuth), it is possible to synchronize groups from the identity provider to MediaWiki groups. There are two built-in group synchronization algorithms, syncall and mapped, described below. It is also possible for an extension to provide additional custom group synchronization algorithms." This more flexible functionality replaces the earlier approach that used that hook and allows a more consistent approach to group management between the plugins.