As per https://github.com/discourse/discourse/commit/d93586c63f257b658fc72e4f9e4edbd1276a140f, the allowfullscreen iframe attribute is not stripped out anymore so fullscreen buttons should show up as expected:
Note that you’ll still need to add the source domain for these iframes to the
allowed iframes site setting. There should be no need for this plugin in the future but thanks to @joebuhlig for making it 