Huh?!?
I use multiple addresses a lot, and recall having used them through ifconfig for years, probably well over a decade. Anyway, at least on Debian, the package net-tools brings ifconfig it back, again with full multiple addresses support.
# uname -v
#1 SMP Debian 5.4.19-1 (2020-02-13)
# ifconfig
enp2s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.10.42 netmask 255.255.255.0 broadcast 192.168.10.255
inet6 fe80::2e4d:54ff:fed8:7b59 prefixlen 64 scopeid 0x20<link>
ether 2c:4d:54:d8:7b:59 txqueuelen 1000 (Ethernet)
RX packets 1162942 bytes 1627138668 (1.5 GiB)
RX errors 0 dropped 42049 overruns 0 frame 0
TX packets 546159 bytes 41181499 (39.2 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 16 bytes 818 (818.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 16 bytes 818 (818.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
# ifconfig enp2s0:1 192.168.1.42
# ifconfig
enp2s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.10.42 netmask 255.255.255.0 broadcast 192.168.10.255
inet6 fe80::2e4d:54ff:fed8:7b59 prefixlen 64 scopeid 0x20<link>
ether 2c:4d:54:d8:7b:59 txqueuelen 1000 (Ethernet)
RX packets 1163011 bytes 1627147417 (1.5 GiB)
RX errors 0 dropped 42089 overruns 0 frame 0
TX packets 546185 bytes 41184061 (39.2 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
enp2s0:1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.1.42 netmask 255.255.255.0 broadcast 192.168.1.255
ether 2c:4d:54:d8:7b:59 txqueuelen 1000 (Ethernet)
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 16 bytes 818 (818.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 16 bytes 818 (818.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0