Overflow is a plain text Craft field type, with a soft or hard character limit. It's perfect for things such as metadata titles and descriptions, which have an "ideal" length, but not an absolute limit.
Overflow will gently inform your content authors when they exceed a soft character limit, but will still allow the entry to be saved.
Opting for a hard limit causes Overflow to behave exactly like the built-in plain text field.
Overflow requires Craft 3.0.0+.
You can install Overflow via the Craft Plugin Store.
If you're a 1337 hacker, you can also install it via Composer, as follows:
$ cd /path/to/project
$ composer require experience/overflow
Overflow adds a new "Enforce character limit" checkbox to the plain text field settings.
Check the box to enforce the character limit (the standard plain text field behaviour). Leave the box unchecked for a soft character limit.