Shield master still has usefulness as if your a tank and you have an ally with booming blade.
1. Ally attacks melee enemy with booming blade (I have a Sorcerer melee fighter who uses booming blade with twin and quicken spell and Shadow blade, sentinel, so this is a likely thing for my group.)
2. Tank moves forward and attacks, then bonus action Shield Master pushes enemy back 5ft.
3. On the NPC enemies turn it moves forward to attack the two and takes booming blade damage or it stands there useless. Which results in the two of you repeat this again until it dies without a chance to fight back. Either way you win, Hurray!!
Team work for the win.
Good plan for players dealing with GMs who don't mix up enemies only sending melee enemies (something I have seen from story GMs who don't care about tactical combat enough to really design encounters) which will result in more satisfying games with varied enemies or because you walk all over enemies like a boss.
WIN , WIN