Is R.E.P.O. crossplay?

No. R.E.P.O. has no crossplay — you can only play with people on the same platform.

Does R.E.P.O. have split screen?

No. R.E.P.O. has no split screen — every player needs their own copy and their own machine.

R.E.P.O. supports 1 to 6 players, and it can be played solo.

Six. That has not changed since launch, whatever you may have read.

Is R.E.P.O. crossplay? — No. R.E.P.O. has no crossplay — you can only play with people on the same platform.
Does R.E.P.O. have split screen? — No. R.E.P.O. has no split screen — every player needs their own copy and their own machine.