Strikethrough is a formatting decoration applied to characters. Therefore you find it in character style definition or in Format>Character command for direct formatting.
The decoration is configurable in the Font Effects tab of the aforementioned dialogs.
Strikethrough is an explicit “decoration” requested by user. It is drawn over visible characters and is not intended to flag consideration on document structure, like deletion, edit or authoring.
If your goal is to review a document so that it meets external constraints like maximum text size, decorating your text with strokes is not what you need.
Enable Edit>Track Changes>Record (and Edit>Track Changes>Show) in the “raw” text (the version you received from an author or the final version you wrote before considering constraints).
The bottom status bar shows the current size of the document. When you press Bksp or Del to delete text, erased characters turn brown with strikethrough and character count is decremented in the status bar. What you add turns gold underlined and character count is incremented in the status bar.
Colours are customisable in Tools>Options, LO Writer>Changes.
If you want to see the final text without the change feedback, untick Edit>Track Changes>Show. Reenable it to see again change information.
You can keep change information if you want. You can also selectively delete it with Edit>Track Changes>Manage to accept or reject individual changes.
PS: when asking here, always mention OS name, LO version and save format. There are subtle differences between platforms (e.g. Tools>Options is the Preferences menu under MacOS). Most recommendations given on this site are only valid when document is saved .odt; other formats need translations and not all features map one to one and frequently need approximations (with corresponding loss of information/formatting).
