Try to report this issue at
https://trac.videolan.org/vlc/
VLC could preserve the support for txt subtitles. Probably it is not a special subtitle format. Is it just the wrong filename extension for SubRip srt or MicroDVD sub subtitle format? I had some experience with the txt subtitles in the past. I think the idea of txt filename extension came from Poland within a media player used in that area. Txt subtitles are still supported by hardware media players (set-top box, tv, ...). Users could already have burned txt subtitles on their cds/dvds so if old VLC versions can play them automatically, new VLC should play them, too. Now they can be loaded only manually (not drag&drop, but VLC menu: Subtitles > Add Subtitle File...).