SO I’m on the latest IDE 1.8 build, and the latest AVR core installed, but seeing this:
Warning: platform.txt from core 'Arduino AVR Boards' contains deprecated compiler.path={runtime.tools.avr-gcc.path}/bin/, automatically converted to compiler.path=/usr/bin/. Consider upgrading this core.
Archiving built core (caching) in: /tmp/arduino_cache_635458/core/core_arduino_avr_uno_b83454a18c6b3814c622b97a97991fe1.a
Hi @outbackhut. This message is produced by arbitrary changes made by the maintainer of the 3rd party package you installed Arduino IDE from. It will not occur if you instead use the official build of Arduino IDE provided by the Arduino company:
https://www.arduino.cc/en/software/#:~:text=on%20GitHub.-,Legacy%20IDE,-(1.8.19)
It’s only popped up today, I downloaded it off the Arduino website itself. Only place I will EVER download the IDE from.
What do you see if you run the following command from the terminal while Arduino IDE is running?:
ps -C java -F
daniel@daniel-Latitude-5300-2-in-1:~$ ps -C java -F
UID PID PPID C SZ RSS PSR STIME TTY TIME CMD
daniel 50817 50807 5 2373909 857388 4 20:33 ? 00:01:29 java -DAPP_DIR=/usr/share/arduino -splash:/usr/sha
daniel@daniel-Latitude-5300-2-in-1:~$
It definitely looks sus. The command I provided was not ideal because it caused the output to be truncated. Please try again with this modified command that will show the full command line of the process:
ps -C java -f --width 9999
I just restarted to this:
I pressed yes and now the issue isn’t coming up when I compile anymore 
daniel@daniel-Latitude-5300-2-in-1:~$ ps -C java -f --width 9999
UID PID PPID C STIME TTY TIME CMD
daniel 62678 62666 99 21:32 ? 00:00:21 java -DAPP_DIR=/usr/share/arduino -splash:/usr/share/arduino/lib/splash.png processing.app.Base
daniel@daniel-Latitude-5300-2-in-1:~$
I still believe you are using an installation of Arduino IDE from a package repository.
In the build of Arduino IDE distributed by Arduino, the JVM is bundled with the Arduino IDE installation. So, for example, on my system where I installed Arduino IDE under /home/per/arduino-ide, I get this output:
$ ps -C java -f --width 9999
UID PID PPID C STIME TTY TIME CMD
per 47039 47033 14 02:23 pts/1 00:00:22 /home/per/arduino-ide/java/bin/java -DAPP_DIR=/home/per/arduino-ide -splash:/home/per/arduino-ide/lib/splash.png processing.app.Base
Note the path of java under /home/per/arduino-ide.
Since the package will create a desktop entry etc., you might end up in a situation where you have the official build of Arduino IDE installed, but don't realize that you aren't actually using that installation.
When you installed the Arduino IDE you downloaded from the "Software" page, did you install it in the /usr/share/arduino folder?
I downloaded the .tar.xz from https://www.arduino.cc/en/software/, extracted it, and ran install.sh.
I’ve run the uninstall.sh and rerun install.sh just in case