I guess that’s true. Just wanted to update via TWRP and was greeted with the Stock Recovery. I did the previous Update to Android 9 with TWRP but probably booted into the OS right away.
So, I flashed TWRP via ADB/Fastboot, updated the OS and Open GApps, rebooted to bootloader and my FP is now running on 21.05.0 and I still have TWRP!
Also, I found one of the reasons why an update of Open GApps is advised: ✏ Update Guide for a modified FP Open OS (with Gapps, Xposed and more) (first paragraph)
I don’t remember having bootloops but rather terrifying battery drain (from 100 down to 0 in minutes) and overheating caused by defective Open GApps after an OS Update without updating GApps. I’m pretty sure this was also discussed here in the forums but I couldn’t find it. Maybe somebody stumbles upon it and can add the link. 