Using classic Outlook for Windows in business environments
So I don't know if this will work long term or for everyone but here is my work around.
Pressed windows key
under recommended apps it showed outlook classic as frequently used
I went to my task bar and removed the new outlook from the task bar. (right click then remove from taskbar)
Then pressed windows key again, right clicked on outlook classic and then clicked on pin to task bar.
Since then when i click on outlook icon on task bar, the classic outlook opens.
Hope this works for you.