So, I saw something that said if you kill a certain character:
Toriel
and reset before saving, you'd still not be blocked off from any endings in the game, including the true pacifist ending, even if Flowey calls you out on it.
I'd really like clarification if this is true or not, and if I have to use the special method to actually delete the memory to see the best ending.
Best Answer
Your source is correct. Flowey remembers that you killed that character, but only mentions it once, and it doesn't affect your ability to reach the True Pacifist ending in another playthrough. This is true of any character in the game that you can kill--if you revert your save to a point before you kill them, for the purposes of reaching the True Pacifist ending, it will be as though you never did so.
The only thing which will permanently alter the state of your game is
which can still be reversed by deleting your save file or uninstalling/reinstalling the game.