Less so in my view. I've designed enough challenges where tension runs high enough to feel like life-or-death but where the stakes are something else entirely to know someone can run a perfectly intense game without killing PCs. Or fudging.
Or without PCs being in danger of death? Is there always something else on the line they really care about?
"Illusionism" is one of those words that demands a specific definition because it can mean different things.
Yep, but unlike 'immersion,' it has one. It's when the player is presented with choices, options, & checks that actually mean nothing - regardless, the results are already decided. But, the player isn't given enough information to divine that fact.
'Placebo rolls' behind the DM screen, the classic magician's force, fudging a die roll behind the screen - all 'illusionism.'
It's a newish game-theory label for a classic (almost ubiquitous back in the day) DMing style. 5e harkens back to those days in a lot of ways, so I find it a good answer to the OP's question...
At its heart though, it strikes me as dishonest
Nope. It's just limiting the information you provide the player, no more dishonest than not showing them the map of the dungeon. That's the problem with Game Design Theory labels, they'll strike people all wrong.
I'd like to see a proposed solution to that scenario. Maybe there's an angle someone has figured out.
How do you avoid killing PCs without either illusionism or taking even the appearance of deadly danger off the table?
It's the 5e forum so 'run something with less of a tendency to drop random/unintended/pointless character deaths in your lap' isn't an option.
OTOH, modding rules is entirely kosher in 5e. So if the issue seems to be sudden deaths from damage spikes in non-deadly encounters at low level, increase low-level hps, or increase the instant death threshold to -(CON+max hps) it'll make a big difference at low level, hardly matter at high. If it seems to be players doing stupid stuff, institute Common Sense Saves.

etc...