Some publications discourage it; that's their "house style" and you should ALWAYS comply with the submissions guidelines for any publication (on or OFF Medium) when submitting work to them.

I've run into this, as well. It's all about consistency and how it looks on the publication's own layout. Others may want specific text in the Kicker.

Medium offers that formatting feature; I see no evidence that they either encourage or discourage its use.

And you're very welcome. :)