It's possible to get all the DLC for every Xbox 360 Forza game even as they're de-listed (Motorsports 2, 3 and 4, as well as the first Horizon), although through unconventional means, for as long as you RGH it (which isn't simple, really, and once your console is RGH'd, watch tutorials from Modded Warfare to see how you can install DLC for Xbox 360 games on a modded console). As for the Xbox One games leading up to Horizon 3, I don't think there's another way to get them.
That, and you only have until September 15th to buy Motorsport 7 and all of its DLC before it's completely gone. As for Horizon 4, you have one more year left before it suffers the same fate as the older titles. If your PC or laptop is good enough to run Motorsport 7 then that's good enough for you to play in if you don't have an Xbox One or Series X/S.