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Hmm, found a Reddit post that says this MIGHT be an indication that you're on a metered WiFi. https://www.reddit.com/r/MotoX4/comments/b20eo0/wifi_logo_with_r/
Check the settings for your WiFi network and see it it's set to metered? I can't duplicate the R on the symbol on my Tab S3 on Pie, Nexus 6p on Oreo or my Pixel 2XL on Android 10 but maybe Motorola has added something?
This doesn't appear to be a new issue. Most of the posts I've found go back to 2017 however. Many of them mention a Beta version of Android, I'm assuming, and most seem to refer to Moto's. Almost all of the threads abruptly end without resolve although half way through the second one listed below a user mentions that Factory Reset cured their issue. I'm not suggesting that but it's obviously up to you how far you want to take it for a cure. It doesn't appear to have any adverse affects other than curiosity and annoyance.
The two threads you linked both show the R in the data/cell triangle, not the WiFi pie. Given that it isn't acting any differently except the icon, I'm going to ignore it.