SwiperFormatterStyle::renderFields() must be public

That's a weird error because base class' method is protected indeed. I took a quick look for 10.2 and 8.8 versions:
https://git.drupalcode.org/project/drupal/-/blob/10.2.x/core/modules/vie...
https://git.drupalcode.org/project/drupal/-/blob/8.8.x/core/modules/view...

So what's your version of Views/Drupal core?

That's a great question, sorry for not including this info in the issue summary. I'm on 9.5.11, but that's even "protected" in that version of Drupal Core. Even more strangely, I didn't get this error in one of our lower environments, even though all environments are running the same Drupal version.

For me, something must have gotten knocked off kilter in my PROD environment, which has renderFields() defined as a "public" function in PROD, though it's clearly supposed to be "protected". Thanks for helping me troubleshoot!

No problem, glad if I can help. My first thought may be - is there anything (contrib or custom module or theme) in your environment that is extending StylePluginBase or even SwiperFormatterStyle instance (this in some kind of custom code)?