Obviously, you did not read the release notes about version 6.
Excerpt from the link above:
Temperature no longer on the web interface
During the development of v6 – we took a decision to remove the temperature from the web interface. It was never consistently displayed on every user’s interface – sometimes we couldn’t get the correct sensor reading, and sometimes we couldn’t get a sensor reading at all. We would find ourselves spending a lot of time debugging issues with sensors for very little gain.
There are other tools that are better suited for monitoring servers and hardware in general, and really an ad-block DNS server is not the place for that. We kept the CPU and RAM usage stats because they are arguably much more meaningful to the operation of Pi-hole than knowing what the temperature is.
Some enterprising users have created web interface mods already to add the temperature back, and you’re more than welcome to run them, but keep in mind the standard Pi-hole upgrade process will stash any modified files on the web interface – so you’ll need to apply them again manually. (If anyone has ever used the speedtest mod – you’ll be used to this…)