MDEV-26278 only considers derived tables having GROUP BY expressions. But if a derived table has SELECT DISTINCT this can also be considered as if table has a unique key on selected fields, and so can be a subject to elimination:

select t1.* from t1 left join  (select distinct a from t2) D on D.a=t1.a;

Also MDEV-26278 does not implement elimination for derived tables with UNION, though it seems possible:

select t1.* from t1 left join 

  (select distinct a from t2

   union

   select distinct a from t3) D 

on D.a=t1.a;