Hi, before I upgrade to ZBrush 2020.1.1, I wanted to make sure that the files can be read by people who are still on ZB2020. Thanks
Files from updates within the same major version number(2020.x) are generally compatible. Previous versions (2019.x, 2018.x 2017.x etc) are generally not backward compatible, and will not be able to open zbrush files created in a newer version (generic formats like .obj are usually fine).
To the best of my knowledge, your files should be compatible.
If not in the middle of an important project, anyone still running an obsolete version of Zbrush should upgrade if at all possible. Not only are they missing important fixes and new features, but older version are not officially supported. You want to be running the most recent version before you report any issues.