It ultimately depends on what the client is comfortable paying for. If all they want is someone to design a website for them and that's all they are willing to pay for, you aren't responsible for anything that happens to it after you've handed it over to them (get this in writing!). I would recommend bringing up some sort of support or maintenance plan, or at the very least bring up the risks associated with not keeping the site up to date.
If they aren't willing to pay for support and/or maintenance, or simply can't afford it, there are alternatives out there that will at least help keep things updated. Services such as ManageWP will update plugins on a set schedule for instance (free), and of course I'd throw in Wordfence for security (also free). There are also a number of free back-up plugins (we use a paid version of Updraft Plus, but the free version should suffice). For a basic brochure site with 5 or so pages, and reliable hosting of course, I think the website would be okay in this instance.
Larger websites, such as 20+ page business sites or eCommerce sites absolutely need some sort of support or regular maintenance. Especially eCommerce.
In the end, it's up to the client. Whatever you both agree to, get it in writing! If they decide to take care of it themselves and **** hits the fan a few months down the road, they can't blame you if you both agreed to the terms and have it in writing.