I figured for source authors, it's easiest saving a debug file in the same folder as whatever .src/.inc you'd be testing without having to manually change paths so there's less folders open at once.
I've made a way to route debug files to Mp3tag's sources folder in any version of Windows be it portable, standard/installed or the Store app:
OutputTo "DEBUG"
SayFormat "$ifgreater($strstr($getenv(mp3tagapp),35795FlorianHeidenreich),0,$getEnv(USERPROFILE)\AppData\Local\Packages\35795FlorianHeidenreich.Mp3tag_rf0p6xgxmspcc\LocalCache\Roaming\Mp3tag,$getenv(mp3tagappdata))\data\sources"
debug "on" "%debug%\IndexDebug.txt"
mp3tagappdata doesn't return the expected value for the Windows app version, hence the initial format string check as a work-around.
I don't have a Mac so I'm looking for help from anyone that does what's the best way of changing that formula to use that version's path at ~/Library/Application Support/de.mp3tag.Mp3tag_<some number>/drive_c/users/Mp3tag/data/tools. I'm not sure if $getEnv() is available nor what strings are usable.
Totally unsure of Linux, from what I've read it's using Wine which is effectively a Windows emulator(???) and clueless where any configuration folders would be saved.