It's been years, but I recall that, for example, when you know every piece of plaintext starts with "https://www https://www", or perhaps know the full contents

It's been years, but I recall that, for example, when you know every piece of plaintext starts with "https://www https://www", or perhaps know the full contents of particular messages, it may become some degree easier to brute force your way to a key. I don't think it's a concern for standards in broad use, more like something you would worry about if you were cooking up your own cipher.