A very significant chunk of games here have de facto linear plots - the average platformer or beat-em-up or suchlike for ex doesn't exactly have an awful lot of story branching. By your logic those should be tagged too which would render the definition broad enough to be essentially meaningless.Whatever you say, it is convenient. Unless you want to petition moderators to add linear tag it it will stay that way.
And a great many RPGs have fundamentally linear plots too, the player just usually has leeway in which order to do some things and what side stuff, if there is any, to explore.