That is, it would likely be necessary to implement functionality similar to Augmented Steam + IsThereAnyDeal through a browser extension (Mozilla Firefox for me).
The problem is that when I'm on store.steampowered.com, I don't know whether the game I'm on has been added to my watchlists. To do this, I need to open the appagg.com website in a new tab, search for the game, and only there, based on the appearance of the button, will it become clear to me. A single icon on Steam indicating that the game has already been added would be much more convenient.
Of course, it would be even more convenient to display which specific watchlist the game is in, or even add games to watchlists directly on the Steam website, but for now, the minimal functionality described above would be sufficient.