Recently, the developers of Diablo 4 gave an interview to the author of the YouTube channel Raxxanterax. During the conversation, they touched on the topic of the loot filter — players have long been asking to introduce it into the game.
As it turned out, it (technically) is already present in Diablo 4, but for now it is hidden, "working behind the scenes."
The developers are considering introducing a full-fledged (familiar from other games) filter, and are currently studying the results of their current system:
We have been actively [working on a loot filter]... well, since about the release of Loot 2.0, or, more precisely, Smart Loot in Diablo 3. The game kind of filters something for you itself. You could say that the game has a kind of built-in loot filter. It just "works behind the scenes" so that you get loot and stuff. This is a kind of process that allows us to understand what we really need from this kind of thing.
Then the developers were asked if they would offer more space for loot (additional stash tabs) — they answered "stay tuned."