I haven't found a way that netplan displays a version number itself, however you can query the package system to find out.
You can check the available/installed packages as provided in this answer:
https://askubuntu.com/a/340536/862145
It is important to know that the package name is netplan.io
user@lab:~$ apt policy netplan.io
netplan.io:
Installed: 0.104-0ubuntu2~20.04.2
Candidate: 0.104-0ubuntu2~20.04.2
Version table:
*** 0.104-0ubuntu2~20.04.2 500
500 http://archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages
100 /var/lib/dpkg/status
0.99-0ubuntu1 500
500 http://archive.ubuntu.com/ubuntu focal/main amd64 Packages
user@lab:~$
Installed shows you the version you have installed, or lists (none)
Candidate lists the version available via apt install
This is from this question:
How can I check the available version of a package in the repositories?