Canon 5DS Manual Focus Assist in Live View

That functionality is only available when using the optical viewfinder (not in live-view mode).

To use it:

1. Put the lens in Manual Focus mode
2. Look through the optical viewfinder
3. Half-press (and hold) the shutter button
4. Manually rotate the focus ring

The AF point will briefly blink red and you may hear the camera chirp (if sound is enabled) when it detects accurate focus at the selected AF point.

Since you mentioned Live View and highlighting, I think you're asking about focus peaking. There are a handful of questions and answers here that discuss focus peaking by name, although not with your camera specifically.

The 5Ds does not offer focus peaking natively, and at this time, I don't think there's any way to add it. There is a piece of open-source software called Magic Lantern that offers this feature to some Canon DSLRs but as of November 2020, the 5Ds and 5Ds R are not among the supported models (these two are listed under the heading "Porting Started" with the note "Hello World," which is basically the same as being not yet available). I have not used Magic Lantern on any camera, so I'm not advocating for or against it, but it does have its own tag on this site.

dSLRs generally do not do focus peaking, which is the feature you describe. It's a feature most often found on mirrorless cameras, where the electronic viewfinder can display it as well as the back LCD display.

The only live view manual focus assistance features you have on a Canon dSLR are magnification on the LCD display; and in the viewfinder the AF confirmation light (the green dot) lighting up.

Focus peaking in liveview is a feature that can be added to Canon dSLRs that can use Magic Lantern, but the 5DS is not supported by ML (too new).