Hello, I currently working with Playmaker in an old Unity project. I am currently going through an iterative Unity upgrade process in upgrading this game from Unity 2017 to Unity 6. Right now I am working on the initial upgrade for taking this project to Unity 2018.4, and I need to upgrade the game's Playmaker version. 
I saw that on the Wiki that Playmaker supports Unity 2018.4+. However, I noticed that on the Unity Asset Store, it states Unity 2021+. I also noticed that the Wiki has not been updated in over 4 years now, and it appears that the info could be out of date. When I download Playmaker from the Unity Asset Store, I get an error that the installer is only compatible with Unity 2021+. 
The Wiki does mention that there are different versions of playmaker that are downloaded depending on the version of Unity you are running currently, and that there is a related bug where a user needs to open an old version of the Asset Store. However, I have not been able to get these instructions to work. Is there a way I can get a version of Playmaker that is compatible with 2018.4?