SLIGHT FIX: by using Shift-Return after most typed-out URL, I'm able to get that URL to remain visible text (in rich text format) instead of auto-turning into a Preview. When pasting in a copied URL, it's a little trickier, I have to first put a couple of spaces on the fresh new line and then either type a space after that pasted URL, and/or do the Shift-Return. It's intermittent, so I'd be curious if others are finding a more solid work around.
Definitely adding my vote to request to simply type a URL in a rich formatted email and keep that URL as a URL, not turn it into a preview without my permission. As it's been on the Mac since the dawn of time. This will be one of those improvements that Apple regrets and at some OS update soon, it will be an option that can be clicked on or off in Mail preferences.
And thanks, @Barney, for the link to Apple's Feedback link. I've reposted it here, and it will hopefully not turn into a Preview.
Feedback - Mail - Apple