I have a plugin named threshold alpha 2 that does it. You can download it here.

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Depending on your OS put the binary in
Windows C:\Users\(USERNAME)\AppData\Local\gegl-0.4\plug-ins
Linux ~/.local/share/gegl-0.4/plug-ins
Linux (Flatpak includes Chromebook)~/.var/app/org.gimp.GIMP/data/gegl-0.4/plug-ins
Then Restart GIMP and
if on GIMP 3
/ search for threshold alpha 2
if on GIMP 2.10 to go the GEGL operation menu and look for "threshold alpha 2" GIMP 3 also has this option. Then apply the filter, the higher the slider the less soft edge excesses