The support for autodetection of txt subtitles has been dropped in VLC 3: https://trac.videolan.org/vlc/ticket/21462

txtsub1.lua = Load txt subtitles for video. The first script tries to load txt subtitles for a playing video (an example: "D:\film.avi" <-- "D:\film.txt"). You should find them loaded in VLC menu where you can enable them or use hotkey "v".
txtsub2.lua = Load video with txt subtitles. The second script should add&play another video into VLC playlist, but this time with VLC option ":input-slave" automatically loading txt subtitles with the same filename.

Activate the Extension in VLC menu during playback of a video:
* Windows/Linux: "View > Load txt subtitles for video / Load video with txt subtitles",
* MacOS: "VLC > Extensions > Load txt subtitles for video / Load video with txt subtitles".

INSTALLATION:
Copy the .lua files into appropriate \lua\extensions\ folder (Create directory if it does not exist!):
* Windows (all users): %ProgramFiles%\VideoLAN\VLC\lua\extensions\
* Windows (current user): %APPDATA%\VLC\lua\extensions\
* Linux (all users): /usr/lib/vlc/lua/extensions/
* Linux (current user): ~/.local/share/vlc/lua/extensions/
* Mac OS X (all users): /Applications/VLC.app/Contents/MacOS/share/lua/extensions/
* Mac OS X (current user): /Users/%your_name%/Library/Application Support/org.videolan.vlc/lua/extensions/