The idea of prospero is to avoid SLR duplications, and to foster transparency. You can register your SLR on Prospero's website only if your SLR is carried on Humans or animals.
It is not mandatory. As you experienced, you can have your manuscript accepted without registration. Even in the journals that strongly endorse PROSPERO (such as PLOS or some BMC journals), they commonly mention that they recommend PROSPERO but not regard as a duty.