this is just one addon from spiraloid's toolkit i made to work in 2.9.3.1 you can find the repo on https://github.com/spiraloid/Spiraloid-Toolkit-for-Blender
their project is MIT license so i should include the following notice found at: https://github.com/spiraloid/Spiraloid-Toolkit-for-Blender/blob/master/LICENSE
Install: download as zip unpack the MirrorAllVertexGroups.py go to blender preferences->addons->install select the MirrorAllVertexGroups.py file
in the vertex group context menu (the little arrow in the right side where you select vertex groups in mesh properties panel uner add/remove vertex groups + - butons) in that context menu, click mirror all vertex groups select the corresponding options.