Nope. Maybe. Nope. 
I don’t use arduino, but I do have one laying around so this is what I did:
sudo pacman -S arduino arduino-avr-core
sudo usermod -a -G uucp $USER
Logged out and back in again and it works as expected.
Arduino IDE crashed on startup with java errors if the arduino-avr-core package wasn’t installed.
Either way if adding your user to uucp or chmod isn’t working for you there’s a problem with your system, or what you’re doing.
I’ll happily try to troubleshoot with you.
EDIT:
I didn’t try the serial monitor though.
Just tried the serial monitor and it doesn’t work, everything else does though. 
Exception in thread "EventThread /dev/ttyUSB0" java.lang.NoSuchMethodError: java.nio.ByteBuffer.flip()Ljava/nio/ByteBuffer;
at processing.app.Serial.processSerialEvent(Serial.java:210)
at processing.app.Serial.serialEvent(Serial.java:183)
at jssc.SerialPort$LinuxEventThread.run(SerialPort.java:1299)
https://bbs.archlinux.org/viewtopic.php?id=246768
NEW EDIT:
Got it, serial monitor’s working now.
sudo pacman -S arduino arduino-avr-core
sudo usermod -a -G uucp $USER # log out and back in of course
sudo archlinux-java set java-14-openjdk
It may or may not affect other software written in java. 