What are the implications of failing to cancel my ED visa extension in Thailand?

Bring cash

You are far from the first person to have this happen.

What it sounds like, is that you failed to cancel your ED visa extension before you left the country.

You need to realize that just stamping out does not cancel an extension officially.

What's going to happen is, when you go to cancel the ED extension they will fine you 500 baht per day for every day that you didn't cancel it up to a maximum of 20K baht

It's not technically overstay, insofar as you cannot be banned for it, and your current entry stamp is still valid. It is a fine for failing to follow procedure in canceling an extension.

If you were on an extension based on higher education you could have got an additional year extension after you graduated under the ED Plus program and I don't know why your University didn't tell you that.

Anyway, you won't be banned from the country but you will be fined 500 baht per day.

Take the documentation from your University saying you stopped studying on XXX date to the immigration office that issued you your extension and cancel it officially.

You're not actually on overstay. That's just the only mechanism they have for fining you for failing to cancel your extension. The only thing you should expect is a fine, which is 500 baht per day since you stopped studying, to a maximum of 20,000 baht. There shouldn't be any other issues now or in the future.

You should have qualified for ED Plus after you finished studying though.