Oof I definitely have this problem... Usually the extremely spoilery stuff is just doodles I wouldn't want to post online anyway, but when I have an idea for a more complete illustration with a spoilery concept, it is a struggle, since I would love to post it, but can't
So it's either rethink the idea to make it less obvious, or don't draw it. Not that there are many people who'd care, but still 
There are some stuff that are technically minor spoilers for my comic but I don't mind showing art related to them because they are going to be revealed relatively early (eg. Nate not being a normal human), or because I found a way to portray them that is too vague for anyone other than me to understand?
I mean, certainly, it's possible to make theories off them, but I think that's actually a pretty cool thing and would not mind if anyone did that
Casually dropping hints and foreshadowing in art, yes, pls. I love it when creators do that.
(I think this is how good anime openings work too, btw - the okay one will just literally spoil the events of the whole season, but the good one will show them in a way that fuels the imagination and makes the viewer want to know more.)