Swipe lock has intentionally been removed on GrapheneOS due to security flaws:
We aren't going to be adding pattern unlock to GrapheneOS. It was removed intentionally as it is not reasonably secure.
source: #28
and
Pattern Unlocks are objectively insecure and leaving insecure elements in the system of the OS is antithetical to the scope and aims of the project.
source: #17674
If you're interested, maybe you want to give this paper a read:
Cracking Android Pattern Lock in Five Attempts
https://www.ndss-symposium.org/wp-content/uploads/2017/09/ndss2017_03A-5_Ye_paper.pdf