Modern trombones have been titled "modern" since about 1920s. That's when the latest significant change took place, and the trombone emerged as it looks today. Since the mid 50's there has not really been happening so much in the trombone evolution. The Instruments all became bigger, and heavier. This probably has to do with the Orchestral-evolution of the late 20th century music. The orchestras got much bigger, and the trombones needed to grow a bit to follow the evolution.

Bach Tenor trombone, Model 12

During the late 80's and the whole 90's, the Alto-trombone almost disapeared in Orchestras. Young players had all replaced the Alto with the tenor, and thus also broadening the trombone-section sound tremendously.
In the begining of the 21st century, the alto seems to have a comeback, and also, it seems that some major orchestras tend to go back to smaller trombones when playing older music.

Rath Bass-Trombone, Model R9DST

In the begining of the 1990s, there was a boom of new B/F valves, all with a new way of thinking more "aero-dynamically". The Thayer-Valve was one of the first grand inventions in this field.

Also The mouthpieces did follow the hype of gaining weight. Since 1995, several companies offer custom-made variations of the standard trombone moutpieces - with added weight. This makes the Trombone sound even heavier and duller. It is easier to play, but doesn't blend in into ensemble-playing as well as the older models.