TextEdit is in the /Applications folder on both High Sierra and Mojave. What is different is the former shows TextEdit among other applications in the Launchpad display, and hidden away in the Other folder on Launchpad on Mojave.
TextEdit will allow you to roll your own features such as Strike-through. Here's mine:

It begins with you entering some text. Then press ⌘-T to bring up the Show Fonts panel. On the toolbar of that Fonts panel, there are icon select buttons, and the one controlling strike-through is obvious. If you want a default strike-through, the just select Single, and the selected text in TextEdit will get a strike-through. If you want a strike-through with color, then on that Show Fonts : Strike-through menu, select color first from the color wheel, and then select Single. Select the text in TextEdit.
Now, on the Paragraph Styles menu that I provided above. choose Show Styles… , and a sheet will drop down as in the following:

Click Add To Favorites, and another sheet will replace it:

Give your new strike-through paragraph style a meaningful name. You can see some of my naming conventions above. I leave the two checkboxes unselected, as I do not want to impose any font information on future selections. Click Add, and you are done.
The next time that you want a strike-through, select the target text and select the appropriate strike-through from the paragraph styles menu.
Wondering how to assign a keyboard shortcut to your new strike-through style? Use System Preferences : Keyboard : Shortcuts per the following screen capture, by adding + TextEdit, and the exact name that you gave your strike-through style:
