Thanks for reporting back, and glad to hear it doesn’t allow it for you. How did you try to set it?
But I just did this on my hEX S with v7.2rc7
ubnt@ubnt:~$ ssh demo@192.168.241.94
demo@192.168.241.94's password:
MMM MMM KKK TTTTTTTTTTT KKK
MMMM MMMM KKK TTTTTTTTTTT KKK
MMM MMMM MMM III KKK KKK RRRRRR OOOOOO TTT III KKK KKK
MMM MM MMM III KKKKK RRR RRR OOO OOO TTT III KKKKK
MMM MMM III KKK KKK RRRRRR OOO OOO TTT III KKK KKK
MMM MMM III KKK KKK RRR RRR OOOOOO TTT III KKK KKK
MikroTik RouterOS 7.2rc7 (c) 1999-2022 https://www.mikrotik.com/
Press F1 for help
[demo@MikroTik] > ip address/print
Flags: D - DYNAMIC
Columns: ADDRESS, NETWORK, INTERFACE
# ADDRESS NETWORK INTERFACE
;;; defconf
0 192.168.88.1/24 192.168.88.0 BR-SW
1 192.168.89.1/24 192.168.89.0 ether5-off_bridge_wrk
2 D 192.168.101.87/24 192.168.101.0 ether1-WAN
3 D 192.168.241.94/24 192.168.241.0 vlan241
[demo@MikroTik] > ip address/set address=192.168.89.1/32 interface=ether5-off_bridge_wrk network=192.168.89.1
numbers: 1
[demo@MikroTik] > ip address/print
Flags: D - DYNAMIC
Columns: ADDRESS, NETWORK, INTERFACE
# ADDRESS NETWORK INTERFACE
;;; defconf
0 192.168.88.1/24 192.168.88.0 BR-SW
1 192.168.89.1/32 192.168.89.1 ether5-off_bridge_wrk
2 D 192.168.101.87/24 192.168.101.0 ether1-WAN
3 D 192.168.241.94/24 192.168.241.0 vlan241
[demo@MikroTik] > ip address/set address=192.168.89.1/31 interface=ether5-off_bridge_wrk network=192.168.89.0
numbers: 1
[demo@MikroTik] > ip address/print
Flags: D - DYNAMIC
Columns: ADDRESS, NETWORK, INTERFACE
# ADDRESS NETWORK INTERFACE
;;; defconf
0 192.168.88.1/24 192.168.88.0 BR-SW
1 192.168.89.1/31 192.168.89.0 ether5-off_bridge_wrk
2 D 192.168.101.87/24 192.168.101.0 ether1-WAN
3 D 192.168.241.94/24 192.168.241.0 vlan241
[demo@MikroTik] > ip address/set address=192.168.89.0/31 interface=ether5-off_bridge_wrk network=192.168.89.0
numbers: 1
[demo@MikroTik] > ip address/print
Flags: D - DYNAMIC
Columns: ADDRESS, NETWORK, INTERFACE
# ADDRESS NETWORK INTERFACE
;;; defconf
0 192.168.88.1/24 192.168.88.0 BR-SW
1 192.168.89.0/31 192.168.89.0 ether5-off_bridge_wrk
2 D 192.168.101.87/24 192.168.101.0 ether1-WAN
3 D 192.168.241.94/24 192.168.241.0 vlan241
[demo@MikroTik] > ip address/set address=192.168.89.0/30 interface=ether5-off_bridge_wrk network=192.168.89.0
numbers: 1
[demo@MikroTik] > ip address/print
Flags: D - DYNAMIC
Columns: ADDRESS, NETWORK, INTERFACE
# ADDRESS NETWORK INTERFACE
;;; defconf
0 192.168.88.1/24 192.168.88.0 BR-SW
1 192.168.89.0/30 192.168.89.0 ether5-off_bridge_wrk
2 D 192.168.101.87/24 192.168.101.0 ether1-WAN
3 D 192.168.241.94/24 192.168.241.0 vlan241
[demo@MikroTik] > ip address/set address=192.168.89.3/30 interface=ether5-off_bridge_wrk network=192.168.89.0
numbers: 1
[demo@MikroTik] > ip address/print
Flags: D - DYNAMIC
Columns: ADDRESS, NETWORK, INTERFACE
# ADDRESS NETWORK INTERFACE
;;; defconf
0 192.168.88.1/24 192.168.88.0 BR-SW
1 192.168.89.3/30 192.168.89.0 ether5-off_bridge_wrk
2 D 192.168.101.87/24 192.168.101.0 ether1-WAN
3 D 192.168.241.94/24 192.168.241.0 vlan241
[demo@MikroTik] >
The last two examples 192.168.98.0/30 and 192.168.90.3/30 should not have been allowed.
As far as MikroTik and /31, RFC 3021, @mrz is quite adamant that MikroTik has no plans to support rfc3021 (probably because NIH syndrome?), but want you to use the MikroTik’s non-standard /32 mask (similar to pppoe) ip /32 help. But RouterOS isn’t the only one not to support /31, FRRouting supposedly has problems as well. It seems that /30 is currently the minimum safe interoperable mask for a point-to-point link. Which seem wasteful of scarce ipv4 addresses. Yes, I know we should be using ipv6, but there are problems with that too.