In a 2015 survey of 60 popular websites, I found that 8 sites took you directly to a sign-up form without letting you see the content properly.
However, for the remaining 52 sites, users visiting the site and who were not automatically logged in, they would see a sign-in option on all 52 (100%) of them, and a sign-up option on 38 (73%) of them.
For all those without a sign-up option, clicking on sign-in then showed the sign-up option as well.
This would tend to suggest that direct access to both options is generally preferable, while the singular directly accessed sign-in option is still reasonably common as a route to displaying a combined or switchable form with both sign-in and sign-up option.
There's several reasons for not having both directly accessed:
Firstly, this reduces the options (user decisions) from 2 to 1 on a main page but once the user has clicked the option to sign-in then the subsequent form/page/overlay allows dedicated space for exactly the type of messaging you mention - i.e. 'Not a member? Sign up here.' or 'Already a member? Sign in here.'
Secondly, sign-in feels like an easier task than sign-up so it's something of a dark pattern to display just one option. Sign-in suggests a two line form, while sign-up suggests all the baggage of complicated registration and validation.
And thirdly of course, there's more opportunity to focus on the important message that's trying to persuade you to sign-up. Few sites just title the form 'Sign up' - they want a bigger more attractive call to action to promote the benefit of signing up now that they've got your attention, and especially at a point when you've already indicated your interested in signing up. Typically the message is 'Create your account to...' or 'Sign up for free...' or 'Join X today and start...'.
Unfortunately, this attitude (from a quarter of the 52 sites) is more focused towards getting new sign-ups, and less focused on being helpful to people who've already signed up and just want to sign in. This is despite arguments from some about optimizing their authorization process.
The data and the better placed attitude of valuing existing users points to providing both options.