With patch openbsd28_3.1.patch applied per directions, OpenSSH 3.1 still does not build on OpenBSD 2.8. 
Error is in cipher.c, see following output from make:

lurp# make
===> lib
cc -O2   -DKRB4 -I/usr/include/kerberosIV -DAFS -I/usr/src/usr.bin/ssh/lib/..   -c /usr/src/usr.bin/ssh/lib/../cipher.c -o cipher.o
/usr/src/usr.bin/ssh/lib/../cipher.c: In function `cipher_init':
/usr/src/usr.bin/ssh/lib/../cipher.c:200: void value not ignored as it ought to be
/usr/src/usr.bin/ssh/lib/../cipher.c:210: void value not ignored as it ought to be
/usr/src/usr.bin/ssh/lib/../cipher.c: In function `cipher_crypt':
/usr/src/usr.bin/ssh/lib/../cipher.c:220: void value not ignored as it ought to be
/usr/src/usr.bin/ssh/lib/../cipher.c: In function `cipher_cleanup':
/usr/src/usr.bin/ssh/lib/../cipher.c:227: void value not ignored as it ought to be
/usr/src/usr.bin/ssh/lib/../cipher.c: In function `ssh1_3des_init':
/usr/src/usr.bin/ssh/lib/../cipher.c:280: warning: assignment from incompatible pointer type
/usr/src/usr.bin/ssh/lib/../cipher.c:299: void value not ignored as it ought to be
/usr/src/usr.bin/ssh/lib/../cipher.c:300: void value not ignored as it ought to be
/usr/src/usr.bin/ssh/lib/../cipher.c:301: void value not ignored as it ought to be
/usr/src/usr.bin/ssh/lib/../cipher.c: In function `ssh1_3des_cbc':
/usr/src/usr.bin/ssh/lib/../cipher.c:314: warning: assignment from incompatible pointer type
/usr/src/usr.bin/ssh/lib/../cipher.c:318: void value not ignored as it ought to be
/usr/src/usr.bin/ssh/lib/../cipher.c:319: void value not ignored as it ought to be
/usr/src/usr.bin/ssh/lib/../cipher.c:320: void value not ignored as it ought to be
/usr/src/usr.bin/ssh/lib/../cipher.c: In function `ssh1_3des_cleanup':
/usr/src/usr.bin/ssh/lib/../cipher.c:329: warning: assignment from incompatible pointer type
/usr/src/usr.bin/ssh/lib/../cipher.c: In function `evp_ssh1_3des':
/usr/src/usr.bin/ssh/lib/../cipher.c:346: warning: assignment from incompatible pointer type
/usr/src/usr.bin/ssh/lib/../cipher.c:347: warning: assignment from incompatible pointer type
/usr/src/usr.bin/ssh/lib/../cipher.c:348: warning: assignment from incompatible pointer type
/usr/src/usr.bin/ssh/lib/../cipher.c:349: structure has no member named `flags'
/usr/src/usr.bin/ssh/lib/../cipher.c:349: `EVP_CIPH_CBC_MODE' undeclared (first use in this function)
/usr/src/usr.bin/ssh/lib/../cipher.c:349: (Each undeclared identifier is reported only once
/usr/src/usr.bin/ssh/lib/../cipher.c:349: for each function it appears in.)
/usr/src/usr.bin/ssh/lib/../cipher.c:349: `EVP_CIPH_VARIABLE_LENGTH' undeclared (first use in this function)
/usr/src/usr.bin/ssh/lib/../cipher.c: In function `evp_ssh1_bf':
/usr/src/usr.bin/ssh/lib/../cipher.c:392: warning: assignment from incompatible pointer type
/usr/src/usr.bin/ssh/lib/../cipher.c:394: warning: assignment from incompatible pointer type
/usr/src/usr.bin/ssh/lib/../cipher.c: In function `ssh_rijndael_init':
/usr/src/usr.bin/ssh/lib/../cipher.c:413: warning: assignment from incompatible pointer type
/usr/src/usr.bin/ssh/lib/../cipher.c: In function `ssh_rijndael_cbc':
/usr/src/usr.bin/ssh/lib/../cipher.c:440: warning: assignment from incompatible pointer type
/usr/src/usr.bin/ssh/lib/../cipher.c: In function `ssh_rijndael_cleanup':
/usr/src/usr.bin/ssh/lib/../cipher.c:477: warning: assignment from incompatible pointer type
/usr/src/usr.bin/ssh/lib/../cipher.c: In function `evp_rijndael':
/usr/src/usr.bin/ssh/lib/../cipher.c:494: warning: assignment from incompatible pointer type
/usr/src/usr.bin/ssh/lib/../cipher.c:495: warning: assignment from incompatible pointer type
/usr/src/usr.bin/ssh/lib/../cipher.c:496: warning: assignment from incompatible pointer type
/usr/src/usr.bin/ssh/lib/../cipher.c:497: structure has no member named `flags'
/usr/src/usr.bin/ssh/lib/../cipher.c:497: `EVP_CIPH_CBC_MODE' undeclared (first use in this function)
/usr/src/usr.bin/ssh/lib/../cipher.c:497: `EVP_CIPH_VARIABLE_LENGTH' undeclared (first use in this function)
/usr/src/usr.bin/ssh/lib/../cipher.c:498: `EVP_CIPH_ALWAYS_CALL_INIT' undeclared (first use in this function)
*** Error code 1

Stop in /usr/src/usr.bin/ssh/lib.
*** Error code 1

Stop in /usr/src/usr.bin/ssh.