networking – Uninstalled netplan, yet its works

I have a small server on Raspberry Pi 4, Ubuntu 18.04.5 LTS. I use few service (apache, mysql, pihole). I want to setup an OpenVPN bridge (TAP). First thing I realized I have to uninstall netplan and back ifupdown, because netplan doesn’t support virtual interface.

Server IP address is 192.168.15.28/24 static.

I did

sudo apt update
sudo apt install ifupdown
sudo apt purge netplan.io
sudo rm -rf /usr/share/netplan
sudo rm -rf /etc/netplan

I edited /etc/network/interfaces

auto lo eth0
iface lo inet loopback

iface eth0 inet static
address 192.168.15.28
netmask 255.255.255.0
gateway 192.168.15.1
dns-nameservers 192.168.15.1

The bridge interface doesn’t work. I changed IP to 192.168.15.29 and after that I see strange things. Both IP 28 and 29 I can ping. In 192.168.15.28 working web page (apache) although in interfaces I set 29 not 28. I googled more to uninstall netplan (I think I did)

I did this:

ifdown --force eth0 lo && ifup -a
systemctl unmask networking
systemctl enable networking
systemctl restart networking

systemctl stop systemd-networkd.socket systemd-networkd 
networkd-dispatcher systemd-networkd-wait-online
systemctl disable systemd-networkd.socket systemd-networkd 
networkd-dispatcher systemd-networkd-wait-online
systemctl mask systemd-networkd.socket systemd-networkd 
networkd-dispatcher systemd-networkd-wait-online
# apt-get --assume-yes purge nplan netplan.io

but still I can ping 192.168.15.28 AND 192.168.15.29

ifconfig output:

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.15.29  netmask 255.255.255.0  broadcast 192.168.15.255
        inet6 fe80::e7d2:d506:fb86:6c97  prefixlen 64  scopeid 0x20<link>
        ether dc:a6:32:78:e7:be  txqueuelen 1000  (Ethernet)
        RX packets 5600  bytes 647455 (647.4 KB)
        RX errors 0  dropped 6  overruns 0  frame 0
        TX packets 1872  bytes 1217307 (1.2 MB)
        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 1190  bytes 76894 (76.8 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 1190  bytes 76894 (76.8 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

wlan0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether dc:a6:32:78:e7:bf  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

What can I do to get rid of netplan setup and use ifupdown and interfaces?