I kind of view it in the same category as having feet in an FPS - it's nice to have, and people will appreciate it, but if you have to go out of your way to do its not really an efficient use of your time.  Lots more things you could animate that actually do affect gameplay, right?

In fact, the only recent 2D games I can think of that did this are Ori and the Blind forest, and Dead Cells, both of which were using 3D models to render their sprites

Remember that 2D bone animations exists and makes these things easy and can keep the same look. You get all the 3D animation tools while keeping the 2D look.

For how needed it depends on how large the sprites is and how realistic it is. This has a lot to do with the uncanny valley effect. If your characters are small and a fantasy style you could ignore it. Adding the transitions does improve the quality of the game but can be left for after everything else is done.

Old games used a stand pose and all animations would start there and go back, this reduced the amount of transitions needed.

2 hours ago, JonasPK said:

I honestly cannot tell if any other games use this, and cannot find anything online about it.

There where a few RPGs that did this but mostly JRPGs  with lager art styles.

For detail studies lookup street fighter, the pro fighters recorder every frame and can even tell you that Ken and Ryu fought very different because of the timing in there animations.

Transitions really started to become standard with Playstation 1 so check it's RPGs and JRPGs.

I recommend you look into 2D bone animation, the new 2D animations allow for much better and easier graphics. You can still keep using pixel art with this style if it is what you want.