Complete list of warnings when Boost 1.72 is built with GCC 7.5 with -Wsuggest-override added to cxxflags:

./boost/program_options/detail/config_file.hpp:167:5: warning: ‘bool boost::program_options::detail::basic_config_file_iterator::getline(std::__cxx11::string&) [with charT = char; std::__cxx11::string = std::__cxx11::basic_string]’ can be marked override [-Wsuggest-override]
./boost/program_options/detail/config_file.hpp:167:5: warning: ‘bool boost::program_options::detail::basic_config_file_iterator::getline(std::__cxx11::string&) [with charT = wchar_t; std::__cxx11::string = std::__cxx11::basic_string]’ can be marked override [-Wsuggest-override]
./boost/program_options/detail/value_semantic.hpp:174:5: warning: ‘void boost::program_options::typed_value::xparse(boost::any&, const std::vector >&) const [with T = bool; charT = char]’ can be marked override [-Wsuggest-override]
./boost/program_options/detail/value_semantic.hpp:20:5: warning: ‘std::__cxx11::string boost::program_options::typed_value::name() const [with T = bool; charT = char; std::__cxx11::string = std::__cxx11::basic_string]’ can be marked override [-Wsuggest-override]
./boost/program_options/detail/value_semantic.hpp:38:5: warning: ‘void boost::program_options::typed_value::notify(const boost::any&) const [with T = bool; charT = char]’ can be marked override [-Wsuggest-override]
./boost/program_options/errors.hpp:186:29: warning: ‘virtual const char* boost::program_options::error_with_option_name::what() const’ can be marked override [-Wsuggest-override]
./boost/program_options/errors.hpp:259:22: warning: ‘virtual void boost::program_options::error_with_no_option_name::set_option_name(const string&)’ can be marked override [-Wsuggest-override]
./boost/program_options/errors.hpp:292:22: warning: ‘virtual void boost::program_options::ambiguous_option::substitute_placeholders(const string&) const’ can be marked override [-Wsuggest-override]
./boost/program_options/errors.hpp:346:29: warning: ‘virtual std::__cxx11::string boost::program_options::invalid_config_file_syntax::tokens() const’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:113:14: warning: ‘virtual void boost::program_options::value_semantic_codecvt_helper::parse(boost::any&, const std::vector >&, bool) const’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:133:21: warning: ‘virtual std::__cxx11::string boost::program_options::untyped_value::name() const’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:135:18: warning: ‘virtual unsigned int boost::program_options::untyped_value::min_tokens() const’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:136:18: warning: ‘virtual unsigned int boost::program_options::untyped_value::max_tokens() const’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:138:14: warning: ‘virtual bool boost::program_options::untyped_value::is_composing() const’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:140:14: warning: ‘virtual bool boost::program_options::untyped_value::is_required() const’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:147:14: warning: ‘virtual void boost::program_options::untyped_value::xparse(boost::any&, const std::vector >&) const’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:151:14: warning: ‘virtual bool boost::program_options::untyped_value::apply_default(boost::any&) const’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:154:14: warning: ‘virtual void boost::program_options::untyped_value::notify(const boost::any&) const’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:304:14: warning: ‘bool boost::program_options::typed_value::is_composing() const [with T = bool; charT = char]’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:306:18: warning: ‘unsigned int boost::program_options::typed_value::min_tokens() const [with T = bool; charT = char]’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:315:18: warning: ‘unsigned int boost::program_options::typed_value::max_tokens() const [with T = bool; charT = char]’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:325:14: warning: ‘bool boost::program_options::typed_value::is_required() const [with T = bool; charT = char]’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:337:22: warning: ‘bool boost::program_options::typed_value::apply_default(boost::any&) const [with T = bool; charT = char]’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:355:31: warning: ‘const std::type_info& boost::program_options::typed_value::value_type() const [with T = bool; charT = char]’ can be marked override [-Wsuggest-override]
./boost/program_options/value_semantic.hpp:93:14: warning: ‘virtual void boost::program_options::value_semantic_codecvt_helper::parse(boost::any&, const std::vector >&, bool) const’ can be marked override [-Wsuggest-override]
./boost/program_options/variables_map.hpp:165:31: warning: ‘virtual const boost::program_options::variable_value& boost::program_options::variables_map::get(const string&) const’ can be marked override [-Wsuggest-override]

BOOST_OVERRIDE was introduced in boostorg/config@ffe4e0f.