A character being undead counts as being dead for True Resurrection. It's why Astarion couldn't be brought back to life as a mortal with it in Baldur's Gate 3.
I understand this, which is why I said to ignore
true resurrection. Nonetheless, you had mentioned in your comment that Sammaster had been dead dead for over 200 years, and I just wanted to correct that point.
Why not bring Prince Rivalen Tanthul then?
Or Vhostym?
Or Ghost (the assassin from the Cleric Quintet novels)?
They were all characters just as memorable as Sammaster who got killed off in novels or sourcebooks and PCs never got to face them.
At what point does a setting become oversaturated with characters who can never stay dead?
I only recognize one of those names, but nothing stops anyone from bringing them back. Bring back the Shade Princes, I say. They were fun.
The purpose of villains in a D&D campaign setting is to provide plot hooks for
DMs and to be defeated by
players, not NPCs in a novel. That doesn't mean that novel can't be fun to read, but you should expect it to be a tie-in, not a storyline unto itself.
Also, I disagree that the characters you list had the same recognition as Sammaster. Sammaster is founder, leader, and head of arguably the most iconic FR villain faction of all time. He is also a wizard, lich, and dragon maniac who has many options to return from the dead. I'd find it weird if he never returned, frankly.
I'm sympathetic to those who are upset at lore changes when it affects the
players. I have a player who is very into Arthurian-themed fantasy, and I know she'd rather play as an old style Purple Dragon Knight rather than a dragon rider. That is because those are two completely separate player fantasies that play to two different audiences. In the case of the Purple Dragon Knights, shifting them from Camelot to Dragon Riders is a zero sum game: we lost something to gain something else. In the case of Sammaster, his return is a positive sum game: we only gain plot hooks for DMs, and we lose nothing.