PROBLEM IDENTIFIED! [<:o)] It has nothing to do with Pegasus! It's Google. Now it's just up to my university's IT to remedy it for me.
Today, with one short series of steps I found on the web. I successful linked my Pegasus 4.7 to Yahoo's IMAP server in mere minutes and had full connectivity - all my Yahoo folders and email appeared and functioned within Pegasus.
However, I had a maddening labyrinth to go through to link my personal (not univ.) Gmail to Pegasus with IMAP. You have to go into your Gmail account, go to Settings, then Forwarding and POP/IMAP, select IMAP "on", then follow a link at the bottom of that page to configure the setup, then follow another link to "Other devices" and finally click to another page to choose to turn "Less Secure Apps" ON - and THEN Pegasus can access Gmail's IMAP as well as Yahoo's! The number of steps to do so is absolutely absurd, but it's done now. And it works fine.
The problem is that my university has partnered with Google with their IMAP setup, and the U's user-unfriendly Outlook web interface does not offer this series of steps to select the necessary IMAP options to access my own university account via Pegasus as I've done for 15 years. And nobody will be back in IT until January 4th.
But now, at least I know it's not any problem with Pegasus and simply a new confounding problem created by Google, the ones who keep making the internet more difficult in the interest of needless innovation and monopoly. ("Labels" indeed! [^o)] Just give me folders!)