I have been the owner of this Samsung Galaxy S9 for about 13 months now, and earlier today my battery died. When I turned the phone back on after partial charging, it acted extremely strangely. It rebooted to the secure boot pin screen a few times, and even to recovery mode a couple of times, rebooting about every 3 minutes. But that isn't it; wifi and cellular refused to connect, the screen would not go off if I pressed the power button, the settings app would not open, many other apps barely works, and the device was very slow for the time it was on.
I did the usual troubleshooting steps, clearing the cache in recovery mode, booting into safe mode, uninstalling recently installed apps, scanning with Knox and Malwarebytes, nothing. But, I remembered the day before, inside Samsung Gallery, there was a little text box before my images that said "SD Card has corrupted information, please back up your SD card and consider replacing it." I didn't think too much about it, or screenshot it. Well, to try something, I removed the SD card and booted normally. The only thing I changed that time was the lack of the SD card and voila, it worked fine. Haven't had an issue since, and I honestly think it might be running better.
For my amateur speculation/hypothesis: I believe when Android starts up, it tries to index installed SD cards. The card I am using is an old Samsung Evo 32 GB card, to the tune of 5+ years. Possibly, with the age of the card, there was light corruption in one folder or cell, and Android didn't know what to do with it. I can't find any corrupted files, but I guess Android hung on parsing the microSD card. I just wanted to share my experience and see if anyone had heard of something similar; I sure cannot find anything on it.
TL;DR: A corrupted microSD card in my Galaxy S9 caused Android to lose most function, and bootloop. Usual troubleshooting did nothing, however, removing the card returned the OS back to normal.

 

Interesting, yeah I know the SD card is pushing it's lifespan hard, but it seems to have all the data there. I haven't noticed any issues since that message and the system failure, but nonetheless. The data kept on it was just pictures from devices generations ago, simply a convenient way to externally store and transfer data. Glad it's fixed, just never heard of it.