I guess everyone is going to have their own definition of "abandonment". HB Studios' last patch for TGC 2019 was in December of 2018, which was roughly five months after release. So yes, if lack of ongoing patching equates to abandonment, then yep - TGC 2019 has been abandoned.
My definition for abandonment of TGC 2019 would be when they quit supporting it on their servers. This game would be little more than an interactive demo without server support for "non official courses"... where the vast majority of quality courses reside and multiplayer and Society event play resides.
It's not actively being patched anymore and most everyone who plays it knows that it needs it. I keep holding my nose as I play it in spite of the flaws, but it's still providing me with the most entertainment of the golf games I still play, because the Society structure allows me to play in events in which I feel I have something at stake.