I'll bet the warning is for the "long term" because what you've done is exactly how one is supposed to deal with spacers until final bar height is determined. Then you cut the steer tube and put the spacers on per manufacturer's spec.

Actually, it isn't "the" purpose of the expander plug, it's one of the two purposes. Primary purpose is to gain purchase against the inside walls of the steer tube so that you can tighen/compress the headset bearings. Second purpose is to provide inner support against the clamping force of the stem.

As for your question about headset cone spacers (erroneously called "top cap" throughout this thread - it is nothing more than a spacer shaped like a cone to make a transition from the head tube diameter to the stem diameter. It provides no support to the steer tube, only stack height for the spacer), there are at least three general options available - the 20 mm is considered high. There are zero- or nearly zero cone spacers available and up until recently, 10 mm or so was a normal one.

You should be able to get one wherever your brand of headset is sold - or go to the headset manufacturer's website. This is actually not an unusual issue - the need to lower the bars further than the cone spacer allows.