The subject box on TMP is rendered by your browser; subject box input is regulated by code not visible to your browser when it renders the box.
There is an HTML attribute for that type of input box for maximum length of input. If it were used, your browser could give an alert when you reach the limit, but that behaviour would vary depending on which browser you are using (and the current version).
I'm not a big fan of using that type of function. HMTL was suppsed to be marking for content, not format. Your browser was supposed to take the markings and render it in a way that is useful for you. We have transformed HMTL from content marking to excruciating control of format.
There are reasons for granular control of format. Whether or not those reasons are "legitimate", "beneficial", or "sheeple mind-control techniques" is a lengthier discussion.
I will say that more granular control of the presentation of material requires more data to represent the same content, increasing your bandwith requirement (it is only a bandwidth "need" if you actually need the additional functionality). While nothing (except your choice) limits your browser's ability to intake HMTL and display it however you choose, the more format specification, the harder it is to have a browser that gives you what you want (think of modding without breaking a game with more and more rules). And, last, the more control of behaviour you cede to the browser, the less secure your environment is.