If you have an open canister and it's past the due date get rid of it....if it loos funny...like the powder is in big chunks get rid of it. I have used unopened packets 3-6 months after the due date. I would also suggest not racing on possibly expired stuff...I think you should learn from your gravel experience.

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"see the world as it is not as you want it to be"

Are you Dutch, as your nickname suggests, wildeman? I can understand then what you talk about.
I hesitated a long time last year before I threw away some gels which had expired half a year. Very hard, being a Dutchman myself!

5 years is a lot but up to 2 years would not worry me at all if there is no protein in the drink ( with proetin 1 year beyount out of date is as far as i would go after reading about it ) gels as well 2 years beyont would not worry me.

i recently had a chest infection . i very rarely take any medicine and the one i had was 2 years out of date i reaserched it and while it may have fallen a bit bellow in effectiveness it was said to be perfectly save to take.
so as always the answer is it depends on the ingridients. ie sugar has a very long live span. and while personally i would not go 5 years beyond sale by date it dosnt suprise me you mostly had no issues.
if the drink that made you sick had protein in it than i guess this liklely your culprit as food scientists seem to suggest 12 month over sell by date is the max thats "100%" save.
also still sealed and already opend for a long while has an impact. ie if your opened powder got a bit wet it might go bad before the sell by date...

I have seen powders go bad after a few years. If it starts to get lumps or looks or smells at all funny i'd throw it out. If it has protein in it, just throw it out. You're not throwing out that much value.

A 400g can of Isostar powder is about €6 at Kruidvat, 790g Isostar long energy is about €15.