Asklemmy

I like the idea of splitting timelines if reverse time travel is possible, but it does have some consequences. The biggest one being that it means you can't actually travel back in time. Time travel may even be relatively simple but as it has no effect on the primary timeline you will never be able to change the past as it appears there; travelling back in time simply creates an alternate reality. As far as the primary timeline inhabitants are concerned, you have either died or vanished (or maybe nothing appeared to happen at all) but you have not travelled in time. It also means it's impossible to return to your original timeline as further reverse time travel will only create new alternate timelines, the closest you can get is a timeline that closely resembles your home one.

Another fun approach is that infinitely many alternate timelines already exist (think Many Worlds), travelling back in time simply means you spontaneously form in another world through quantum fluctuations or something equally hand-wavey. The thing I find interesting about this one is that it doesn't necessarily involve time travel at all. You form with the memories of having travelled in time, sure, but you have just spontaneously formed through quantum fluctuations so it's reasonable to assume your memories have too; it may have just been a randomly formed memory that didn't actually happen. Since it's just random fluctuations there'd also be infinitely many universes where you spontaneously pop into existence with no time travel memory, so I suppose in a way this never was time travel. The original timeline would be unaffected by this kind of travel as you can only move to universes where you have already spawned in.

The way I see it the only way to actually change the past in your current timeline arguably involves destroying the universe. You'd have a single timeline and each instance of reverse time travel cuts off your timeline's future and links back to a previous point from which time can continue. You can visualise this timeline as a piece of string, time travel is a loop in that string. If you travel back in time by a year, everything you did in that past year is within that loop off to the side of the primary timeline; the loop starts and ends at the same point. Time travel would essentially delete your future and plonk you back onto the primary timeline. No need to worry about the grandfather paradox; you were born in a loop off to the side of real time so killing your grandfather doesn't change that loop. It works around the bootstrap paradox for similar reasons; the information was created in some loop somewhere, even if it appears to have created itself on the prime line. It's a nice thought experiment but the problem here is that if you travel back in time but fail to change the conditions which caused the time travel you may have just ended the universe in an infinite time loop.

The multiverse conception of time travel is nice and fine for fiction, but in reality, if you think about what such a time machine would have to do, it would have to create an entire new universe, with all precise details exactly in place, and it would have to have a lower entropy than your origin reality. Good luck building such a machine