Not at all, for example you can't implement the ratcheting down semantics of pledge() using seccomp. Say starting with a broader promise set "stdio rpath recvfd", and then dropping to "stdio" after full init.
pledge() can also be found in over 85% of OpenBSD's base system.
There's Capsicum for Linux[1]. Its a port of Capsicum[2] from FreeBSD to Linux. Capsicum was a joint project between the FreeBSD foundation, Cambridge and Google to create a hybrid capabilities framework. But Capsicum allows developers to do the same privilege dropping that pledge does. However Capsicum is more fine grained then pledge so its less easy to use. Also Capsicum for Linux is also out of tree currently.
pledge() can also be found in over 85% of OpenBSD's base system.