When upgrading Playfab API from 2.97.201014 to 2.109.210521 ini Unity 2019.4.26f1 I get
Assets\PlayFabSDK\PlayFabSDK\Shared\Public\PluginManager.cs(9,17): error CS0433: The type 'ConcurrentDictionary<TKey, TValue>' exists in both 'System.Threading.Tasks.NET35, Version=3.0.2.0, Culture=neutral, PublicKeyToken=null' and 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'