There was a Planeswalker named Feroz that died in a lab accident with a fire elemental trapped in ice.
The most interesting part of this was that it's before the mending.

When Feroz died, he perished around ~3980 AR while the last block which has this dating system was Odyssey, at 4306 AR. This was before Time Spiral.

He didn't revive whatsoever, which contradicts most PW's being depicted as immortals that could be beheaded and live (Urza). So one of two things happened.
1. Pre-mending Planeswalkers might not all be immortal.
2. Feroz still lives.
3. My source is incorrect. (I used mtgsalvation because I don't have many MtG merchandise before RtR.)

Perhaps this, as well as Liliana's naivete about Planeswalker powers when the Mending happened, tell that pre-Mending Planeswalker powers needed some level of mastery before becoming active? Feroz himself didn't like summoning and perhaps didn't spend time learning how to master this power or the Planeswalker Spark.

Even oldwalkers were killable. At least four planeswalkers died in Invasion block alone: Tevesh Szat, Daria, Kristina of the Woods, and Urza.

Yes, but Feroz died with something a bit more blunt than Remove Soul. (Wish that was in Time Spiral as a black version of blue's Essence Scatter.)

Then again the other two aside from Urza got killed by Tevesh in a more blunt matter too. Unless I'm failing to remember some things.
I opened up this thread because I got some flak over making a child oldwalker be experimented on and killed by a dead organization in M:EM. I wanted to do some actual research on oldwalkers before continuing with the plot.

I don't think it was ever stated they were unkillable, just nigh immortal beings that don't age, need air, food, water or a physical form.

In the same comic, Serra was killed by being bludgeoned in the head with a stick.
But an important thing to remember is that said source was also pre-revision, before the writing was moved in house and certain concepts consolidated such as the qualities of a planeswalker.

But, you're also laboring under the misconception that I said walkers were immortal, which was not the statement. They are just incredibly hard to kill.

The fact that it's pre-rev material is always important to keep in mind, but I think with a bit of good will, I could see those deaths working under revisionist rules as well.

In Feroz's case, we are probably dealing with an elemental that was noticeably stronger than your average Fire Elemental due to being charged with Ulgrotha's recovering mana, which is what allowed it to break out of the crystal in the first place. The comic explicitly states that the attack was "too quick to raise a defense" and calls the fire "an inferno". Feroz's body was probably harmed too quickly and too badly to put himself back together.

As for Serra, it's not her who gets taken out by a wooden stick, it's the guy who attacks her. Serra actually dies to the black spell he casts on her, and it's made pretty clear that Serra wants to die at that point, so she doesn't heal or defend herself.

Now, the guy who attacks Serra claims to be a planeswalker himself, and I think there are ways in which we could make that work as well. For starters, it's not clear from the comic whether he even dies or whether Brother Angus just knocks him out. Then there's the possibility that he might just be boasting or, as Squirle pointed out in his blog, that it's one of those old school wizards who has learned how to travel to other planes by magic. I like to think he'd have fixed his ugly mug if he was actually a transcendental being with a body of pure energy...

I mean, come on guys, Urza almost got killed by a tin bird eating through his entrails in Time Streams...