Polearm Master has always given a Reaction attack with the Quarterstaff. From the first printing of the PHB page 1658:
Spear now works with it too.
It is however much easier to get someone to trigger that opportunity attack with a reach polearm however.
Unless you keep knocking them away from you with your bonus action, while remaining the person they have to get through to get to anyone else.
Oh right, I forgot that one. But it still does that on it's own, it doesn't have anything to do with shield mastery, or being combined with shield mastery. I'm still perplexed by what the OP expects to happen by involving shield mastery.
Attack, push back, wait, reaction attack if they come back in. Or, attack, knock down, step back, reaction attack if they get up and come back in.
I imagine they are trying to make a hoplite. Or at least thinking about it. Still PAM and UA Spear Mastery would probably do it better.
Yep, that's what I would use it for. And honestly, on a fighter I'd feel comfortable spending the feats to do it. Protection Fighting Style, those two feats, maybe Spear Mastery in the distant future after a couple ASIs or non combat related half feats, on a Battle Master, and you've got basically a 4e style fighter with light control. Especially if you take Goading Strike. Shield Mastery lets you spare your Manuevers for Disarm, Goading, Lunging, maybe Commanders, and Parry.
Sounds fun.
Thing is, most of your bonus action options are situational. They won't always be the best option. I don't need for a bonus action option to be used every single round to be worth it. I need it to be meaningful when it comes up. I
like having multiple options for each of my actions in a round.