The cake is not a lie?

As pointed out in the question,

the cake is shown at the end of the game.

During the final battle with GLaDOS,

a recipe for cake is given by one of the personality cores you destroy. According to the Portal Unofficial Wiki, the recipe contains "inedible or undesirable ingredients".

Finally according to the answer to this Arqade question:

When GLaDOS is defeated, she says the line "There really was a cake".

All this implies that the cake does actually exist. However it remains unclear whether GLaDOS ever really intended to reward Chell with cake for completing the trials.

While it's questionable whether it's canon or not, GLaDOS's song "You Wouldn't Know" from Lego Dimensions implies heavily in its lyrics that the cake in Portal really was a lie.

How? By repeatedly insisting that the cake she has now is real... as if she had lied about it in the past.

Warning: Spoilers for the plot of Lego Dimensions since it's the End Credits theme.

However, this isn't the only evidence. Portal 2 made everything after the Intubation Associate brings you back in the facility non-canonical. If you don't know what I'm talking about, this is part of the ending that was patched into the game in early 2011 in order to bridge Portal 1 with Portal 2.

Other than a few cores, the entire facility is shut down when you kill GLaDOS at the end of Portal 1. You bring GLaDOS and the facility back to life during the events of Portal 2.