A long bob or lob is perhaps the most popular women haircut ever as it’s refined, it fits almost any face shape and it matches any style. How to wear a long bob in 2020 and after it to look stylish and modern and not outdated?
The trendiest idea for now is a shaggy 70s inspired lob with a curtain fringe , at least a side one, and such an option is amazing for a relaxed look. You may go for waves, too, or at least give your hair some texture. If your hair is very straight, forget it and rock your natural straight hair – embrace your natural beauty!

a dark messy long bob with light highlights and a fringe is a beautiful idea if you don't want any waves
As for colors, slight balayage or some highlights will be a great idea, and ombre will do too, especially if you have very straight hair. Stick to more or less natural hair colors and shades and avoid non-traditional ones, a modern lob is about classic beauty and elegance.

a long bob with blonde and honey-colored balayage and waves is a stylish and very modern idea to try

a long bob with icy blonde balayage and blonde highlights plus some volume is a stylish and cool idea

a simple dark long bob with straight parting is a timeless idea to rock, get medium volume and that's it