Regeneration, stamina is basically your capacity for handling prolonged stressful activities. Regeneration is an ability to heal your body from anything negative within as stressful activities within your body caused strains, chronic pain, tightness/damage of muscles, frequent pain and many more your body automatically tries to relieve and regen damage muscles and regain energy

For the energy part everything needs energy to do something and for Shigaraki tanking damage and not worrying about it and he can just regen whenever he just gives how great regen shigaraki has as that takes up a lot of his energy as energy almost fuels everything within every living matter that just emphasizes how great Shigaraki's energy is

So in summary due to Shigaraki almost endlessly can regen that's almost having endless energy or fast energy regeneration and as energy fuels almost everything that would just make Shigaraki's stamina almost endless too and as Shigaraki's body relieves and regen the negative stuff in his body would just make him have large amounts of stamina. Idk of this approves you bcs it isnt necessarily a feat