Underpowered when you consider the problems
The actual benefit of the feat is good—until you consider the context in which you would have to use it in order to get that benefit. Then all of the problems that thrown weapons have rear their ugly heads, and this feat starts looking quite poor indeed, particularly since making the most of it requires you to avoid the only things that can start to help with thrown weapons.
Anyway, my thoughts:
“Throwing Weapons Master” is a very strange name for this effect—it doesn’t do anything with throwing weapons, much less master them, it turns otherwise-non-throwing weapons into throwing weapons, which is rather different.
D&D 3.5e also had a feat that did this, called Throw Anything. Though it only calls out melee weapons as benefiting from the feat, the ability to use anything you can lift as an improvised melee weapon means you really could use it to throw anything. The same is true in D&D 5e, so I suggest that Throw Anything would be a good name.
Remember that even without this feat, you can throw a melee weapon that lacks the thrown property. It just reduces the weapon’s damage to 1d4—a loss, for sure, when you throw a maul or greatsword, with their 2d6 damage dice, but the difference isn’t that large. A d4’s expected value is 2½ while 2d6’s is 7, so this feat effectively gains you +4½ damage. That’s pretty good, but it helps to keep things in perspective.
Throwing a weapon, only to re-summon it in the following round using Pact of the Blade, is a poor plan. You only get one attack every two rounds that way, because it takes your action to re-summon the weapon. Without a way to summon the weapon faster, you will halve your damage from the start, and then do even worse because you cannot benefit from multiattack options. This is even worse than the usual situation for throwing weapons—where you can draw a weapon as your item interaction and throw it in the same round, for 1 attack per round rather than 1 attack per two rounds—and that situation was already largely non-viable.
Unfortunately, there are just not a lot of solutions to this problem. The best answer we currently have on how to optimize throwing weapons focuses on sneak attack as a way to make up for the lack of extra attacks. The only answer we have for more attacks relies on having multiple weapons drawn and in hand before you start (and then, the next round, you’re back down to one attack per round).
There are pretty obvious problems here with both of these: a maul or greatsword does not qualify for sneak attack, so you cannot go the rogue route, and throwing one takes two hands, so we cannot rely on having more weapons already in-hand to enable more attacks. Pact of the Blade, since it requires an action, doesn’t help here at all—an item interaction is still superior.
And this is the real reason why that “+4½ bonus damage” is kind of fictitious—you only get that when using large weapons that will not work very well here. Moreover, since really what we are doing is eliminating a penalty, this feat kind of “puts you back where you should be” rather than “puts you ahead of the curve,” which is important. Your “+4½ bonus damage” doesn’t mean you’re doing more damage than someone who just swings a greatsword, only that you are doing the same damage. Except the person who holds onto their sword can keep swinging it, which means everything.
You could go with rapiers instead, but then you only get a d8 (expected value 4½) instead of 2d6, dropping your damage bonus to +2 instead of +4½. And a +2 bonus to damage on all attacks isn’t bad, but you’re still dealing with the very-awkward situation that throwing puts you in. Even with assassin, it still isn’t good.
Ultimately, the only way it would make a lot of sense to use this feat is to enable a final shot against a fleeing enemy that you cannot, for some reason, catch up with. Then you could throw your weapon, get in more damage than you otherwise would, and the fact that you cannot throw again doesn’t matter (because either you down them, or they escape as they would have anyway). A feat is a huge investment for a situation that won’t come up very often, and even then it only somewhat improves your odds of success in that situation.