Mojave also has the last "classic" version of the Mail.app.
They've made changes in the Catalina Mail.app that makes it impossible to re-create "the old look" of Mail, which I prefer...
I noticed that when you migrate your user data from Mojave to Catalina, the "classic" view of the Mail.app somehow migrates to Catalina as well, so there is a way not to "loose it".