No, that is not normal, it's an unusually high kill rate in my experience. When I've been in parties that hit those levels, there have usually been one or two kills per campaign that require resurrection (though more close saves with resurgences and whatnot).
It may be due to bad player tactics, weak characters, or the GM runs things tougher than the average GM - either "higher CR" or just "better", some GMs (like myself) manage to squeeze a lot more kill per hit die out of opponents. Sometimes a GM runs a harder game, which is fine.
But of course the thing about high level D&D is that you have multiple levels of defense against death, so as long as it's not impacting the fun, there's nothing really wrong with it. You get to play croquet with the angels on a regular basis, heck, that could be part of the plotline (Order of the Stick, anyone?)
If you do want to do something about it, and don't want to embrace character optimization, I would review the results of combats with the rest of the group and see if the deaths were preventable - do party fighters repeatedly charge recklessly into combat, ruining the ability of mages to area effect; do you get spread out all over the battlefield where the cleric can't effectively help you when you're in trouble; do characters not participate well (the tank fighter who hides in the back, the cleric who's too busy fighting to heal)?
Tactics In Depth
I had to coach one player group that was getting demoralized about how tough combats were going in one D&D campaign. In general they didn't have any coordination - they'd open a door and see bad guys. One guy would run in (before the mages could cast damage or battlefield control spells), others would back off, people didn't have mobility or line of sight. They'd decide to run, except most of the group would retreat, but one guy would want just one more round of full attacking, and then be cut off, and then half of the rest of the party would keep fleeing but the other half would come back and help them, and get cut off themselves... I taught them to do simple things like NOT go in the door, but back off and form an inverted triangle around the door and let them come to the party (for dumb melee opponents of course) so that they are the ones getting surrounded and losing actions and being separated. One PC that was taking on a leadership role took it upon themselves to "call the shots" in combat so that a retreat or attack was performed by the whole group. There's a lot of D&D-specific ticky combat tactics stuff, but I always see the greatest difference being made by system-independent "having your crap together" kinds of basic techniques. Have patience, don't get split up, coordinate maneuver to take the fewest attacks but deliver the most, concentrate fire.
I remember in one five year long 2e campaign I ran, the team leader went so far as to run IA drills and basic response plans. When they would come into contact, there were set formations they would move into automatically. The shouted command "Blue" told PCs to close their eyes to avoid the mage's Color Spray. The only PC death in that campaign came from PC on PC action.
Best Answer
The text you quoted defines death attacks (as a keyword); also known as death effects. It tells us what usually death attacks are and which special rules govern them.
However it does not say that all abilities that share some features with death effects are death effects. The Assassin's Death Attack shares some features with the most common death effects, but is not explicitly marked as a death effect.
By contrary, the Death domain granted power is explicitly described as a death effect (so the victim cannot be raised through Raise Dead, and Death Ward protects against it).
So, no. A victim of the Assassin's Death Attack could be resurrected normally by the Raise Dead spell.
That said, the lack of a death effect descriptor to the Assassin's Death Attack could be a minor design flaw that slipped through various editions (3.0, 3.5 and even in Pathfinder). I'd feel confident in house ruling it as regular death effect.