While searching for information about MIXED_PAGE_ALLOCATION in SQL 2016+ and TF1118 I came across great examples about Page allocation changes in Sql Server 2016 here
Summary: For user databases in the versions of Sql Server older than 2016, the first 8 data pages were allocated from the mixed extent and next pages from uniform extent. In Sql Server 2016 even the first eight data pages were allocated from the uniform extent. Below image summaries this page allocation mechanism comparison between Sql Server 2016 and older versions of Sql Server