A plugin Makefile is capable of modifying flags with something like
CXXFLAGS := $(filter-out -std=c++11,$(CXXFLAGS))
CXXFLAGS += -std=c++20
but you must use C++11 if you want to distribute your plugin to other users due to compiler versions used by my build system. Remember that plugins must be built with libstdc++ 5.4.0 on Linux, which I doubt supports C++20. Not sure about C++14 or 17.