This is a list of various interesting security features since v3.4 and when they were introduced in the upstream kernel. Feel free to add anything more!

Version Feature
v3.5 seccomp-bpf, x86
v3.7 PXN, arm64
v3.8 seccomp-bpf, arm
seccomp reported in /proc/$pid/status
finit_module syscall and LSM hook
Version Feature  
v3.5 seccomp-bpf, x86  
v3.7 PXN, arm64  
v3.8 seccomp-bpf, arm  
  seccomp reported in /proc/$pid/status  
finit_module syscall and LSM hook    
v3.13 remove %n from printf  
v3.14 ptdump, arm  
kaslr, x86    
modules ro/nx, arm    
stack-protector-strong    
kexec_load_disabled    
v3.15 seccomp-bpf, mips  
lkdtm WRITE_KERN    
module aslr, x86    
v3.16 harden sysctl writing  
v3.17 seccomp syscall and TSYNC  
request_firmware LSM hook    
v3.18 kernel memory W^X, x86  
overlayfs v3.18    
v3.19 kernel ro/nx, arm  
modules ro/nx, arm64    
ptdump, arm64    
seccomp-bpf, arm64    
PXN, arm    
crypto- module prefixing    
ecryptfs one-byte heap write fix    
arm64 mmap ASLR fix    
vdso ASLR fix, x86_64    
vsyscall=none, x86_64    
vdso ASLR, mips    
v4.0 kernel ro/nx, arm64  
stack ASLR fix    
seccomp-bpf, RET_ERRNO capped to 4095    
v4.1 kernel stack buffer overflow detection, mips  
INET_DIAG cookies fixed    
ET_DYN ASLR separate from mmap ASLR    
v4.3 PAN emulation, arm  
ambient capabilities    
seccomp-bpf, powerpc    
x86_32 direct socket calls    
v4.4 vsyscall CONFIG  
v4.5 ASLR entropy bits sysctl  
v4.6 KASLR, arm64  
RODATA on by default, arm64    
RODATA on by default, arm (ARMv7+)    
RODATA mandatory, x86    
v4.7 LoadPin LSM  
KASLR text, MIPS    
SLAB freelist ASLR    
brk ASLR weakness fixed, arm64 compat    
eBPF JIT blinding    
v4.8 SLUB freelist ASLR  
KASLR text phys/virt split, x86_64    
KASLR memory, x86_64    
gcc-plugin infrastructure    
fix _etext, arm    
fix _etext, arm64    
HARDENED_USERCOPY lkdtm tests    
KASLR with hibernation, x86    
seccomp vs ptrace fixed    
HARDENED_USERCOPY    
NX stack and heap, mips    
v4.9 latent_entropy plugin  
vmap stack, x86    
thread_info in task_struct, x86    
random_page() cleanup    
RODATA mandatory, arm64    
user_ns restrictions    
v4.10 CONFIG_DEBUG_LIST hardening  
PAN emulation, arm64 v8.0    
thread_info in task-struct, arm64    
get_user zeroing fix, arm    
report nnp    
seed RNG from UEFI    
CONFIG_DEBUG_WX, arm64