Checking in in 2026. I have an old project that was last developed in 5.5.0f1 that I was unable to open.
I was able to finally get it open with the last Unity 5 version, 5.6.7f1. If the editor does not open from Unity Hub, try opening it from the exe first. For me this showed the license dialog and ultimately let me open the editor.
For me, on Windows, the editor exe was located in C:\Program Files\Unity\Hub\Editor\5.6.7f1\Editor