Is it possible for the Suicide Mission to be impossible?
Is it possible to play Mass Effect 2 so badly (loyalty, recruitment, etc) that the suicide mission is impossible to complete without Shepard dying at the end?
The definition of impossible in this case is entering the Omega Relay (and therefore passing the point of no return) with a sufficiently bad game state that no combination of decisions can lead to Shepard surviving and the save being importable.
The only criteria for Shepherd surviving is (not sure if you wanted spoilers or not)
How many squadmates are alive. If you have 0 or 1 squadmates after beating the final boss, Shepherd will fail to catch the shuttle out (literally no one there to catch them), and die as the base explodes.
My guess this is a practical limitation --
-- 0 or 1 surviving squadmates means Shepherd can't form a full party of 3, and the game may not be adequately able to handle that -- better, then, to give a game over, rather than dedicating time and programmer hours to bulletproof the game for a tiny fraction of a fraction of games played.
This is actually a bit tricky to pull off in regular gameplay, because
Loyal squadmates taken into the final fight do not die, so if you do take two loyal squadmates into the fight, it is impossible for shepherd to fail -- there's your 2 survivors right there.
The precise parameters of how to achieve this are numerous and varied, so if this is something you desire to plan out, I leave it as an exercise to the reader to plan precisely how to achieve this. I recommend a Suicide Mission flowchart, such as SPOILER WARNING: https://i.redd.it/illia8pzcdd41.jpg .