I've just upgraded 1.8.12 to 1.8.13 on (Debian) Linux x86_64. The Arduino binaries etc. are in /usr/local/lib/arduino/arduino-1.8.13, next to the older arduino-1.8.12. There's oddments in ~/Arduino and ~/.arduino15 as expected.
Using 1.8.12, I've got some extra settings in platform.local.txt found in .../arduino-1.8.12/hardware/arduino/avr/platform.local.txt
If I replicate that file to the equivalent point in the 1.8.13 tree it's ignored.
Looking at IDE output I see
/usr/local/lib/arduino/arduino-1.8.13/arduino-builder -dump-prefs -logger=machine -hardware /usr/local/lib/arduino/arduino-1.8.13/hardware -hardware /home/markMLl/.arduino15/packages -hardware /home/markMLl/Arduino/hardware -tools /usr/local/lib/arduino/arduino-1.8.13/tools-builder -tools /usr/local/lib/arduino/arduino-1.8.13/hardware/tools/avr -tools /home/markMLl/.arduino15/packages -built-in-libraries /usr/local/lib/arduino/arduino-1.8.13/libraries -libraries /home/markMLl/Arduino/libraries -fqbn=arduino:avr:uno -vid-pid=1A86_7523 -ide-version=10813 -build-path /tmp/arduino_build_855625 -warnings=none -build-cache /tmp/arduino_cache_548642 -prefs=build.warn_data_percentage=75 -prefs=runtime.tools.avr-gcc.path=/home/markMLl/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7 -prefs=runtime.tools.avr-gcc-7.3.0-atmel3.6.1-arduino7.path=/home/markMLl/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7 -prefs=runtime.tools.avrdude.path=/home/markMLl/.arduino15/packages/arduino/tools/avrdude/6.3.0-arduino17 -prefs=runtime.tools.avrdude-6.3.0-arduino17.path=/home/markMLl/.arduino15/packages/arduino/tools/avrdude/6.3.0-arduino17 -prefs=runtime.tools.arduinoOTA.path=/home/markMLl/.arduino15/packages/arduino/tools/arduinoOTA/1.3.0 -prefs=runtime.tools.arduinoOTA-1.3.0.path=/home/markMLl/.arduino15/packages/arduino/tools/arduinoOTA/1.3.0 -verbose /usr/local/src/arduino-examples/double-tap-and-hello/double-tap-and-hello.ino
/usr/local/lib/arduino/arduino-1.8.13/arduino-builder -compile -logger=machine -hardware /usr/local/lib/arduino/arduino-1.8.13/hardware -hardware /home/markMLl/.arduino15/packages -hardware /home/markMLl/Arduino/hardware -tools /usr/local/lib/arduino/arduino-1.8.13/tools-builder -tools /usr/local/lib/arduino/arduino-1.8.13/hardware/tools/avr -tools /home/markMLl/.arduino15/packages -built-in-libraries /usr/local/lib/arduino/arduino-1.8.13/libraries -libraries /home/markMLl/Arduino/libraries -fqbn=arduino:avr:uno -vid-pid=1A86_7523 -ide-version=10813 -build-path /tmp/arduino_build_855625 -warnings=none -build-cache /tmp/arduino_cache_548642 -prefs=build.warn_data_percentage=75 -prefs=runtime.tools.avr-gcc.path=/home/markMLl/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7 -prefs=runtime.tools.avr-gcc-7.3.0-atmel3.6.1-arduino7.path=/home/markMLl/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7 -prefs=runtime.tools.avrdude.path=/home/markMLl/.arduino15/packages/arduino/tools/avrdude/6.3.0-arduino17 -prefs=runtime.tools.avrdude-6.3.0-arduino17.path=/home/markMLl/.arduino15/packages/arduino/tools/avrdude/6.3.0-arduino17 -prefs=runtime.tools.arduinoOTA.path=/home/markMLl/.arduino15/packages/arduino/tools/arduinoOTA/1.3.0 -prefs=runtime.tools.arduinoOTA-1.3.0.path=/home/markMLl/.arduino15/packages/arduino/tools/arduinoOTA/1.3.0 -verbose /usr/local/src/arduino-examples/double-tap-and-hello/double-tap-and-hello.ino
Using board 'uno' from platform in folder: /home/markMLl/.arduino15/packages/arduino/hardware/avr/1.8.3
Using core 'arduino' from platform in folder: /home/markMLl/.arduino15/packages/arduino/hardware/avr/1.8.3
Detecting libraries used...
Blank lines inserted to aid readability.
Sure enough, if I copy platform.local.txt into ~/.arduino15/packages/arduino/hardware/avr/1.8.3 things work as expected.
Is that reference to 1.8.3 expected?
MarkMLl