Headline length is one of the crucial aspects in SEO, where the optimal amount of characters is between 48 and 60, the range that Google displays on its search results.
As for the subtitle, I compare this one to the meta description of a website, which is best with a length of in between 145 and 160.
So without any regards to what Medium actually allows us, when I write an article with a conscious title design in mind (which is not always the case, sometimes it's just a title I personally like) I try to implement the SEO rules at this stage.
And even if you use a perfect SEO optimized title here, it won't be SEO optimized for the Google display.
If you check the SEO settings in your story edit, you will notice that Medium automatically adds your author name and | Medium to the Google title. If your title alone already has 60 characters, then that part will get truncated.
But honestly, I have not been too serious with the SEO settings area of my articles. I just follow the general rule when drafting titles and subtitles since I use this rule daily during my line of work as well. I just got used to it.