Old replays usually give message "Obsolete replay" when LFS tries to play them.
For example vMaxA.Tcholakov_AU1_LX4_FINISHED_15.spr from https://www.lfs.net/forum/post/172306#post172306
-> "Obsolete replay"

But sometimes old replays start to play and after a while give error "OOS" (out of sync?)
For example autotestoctober1.spr from https://www.lfs.net/forum/post/1019956#post1019956
-> OOS Error after 50 seconds

I know that the OOS error can also be caused by editing the layout in replays(?), but I found some replays like that now and doubt it was done there..?

That is due to changes in collision system (0.5Z28->0.6B) (?) I believe.
You can see that replay go OOS shortly after he colide with object.
In my opinion it's better that couple of replays go OOS than simply make everything pre-0.6B obsolete because there weren't any other major changes.
Also that kind of OOS will never happen in MPR replay, because they do not run "true" physics.