Does this mean that when I click on tags present in the Metadata block of an issue leads to all the issues related to that tag.

e.g., if a tag specifies easy_fix and if I click on it, it should show me all the easy fixes ? Is this feature to be documented ?

So the feature you're describing is also not documented (while it should) but the feature that @puiterwijk implemented in the PR linked to above is the possibility to search issues having a certain custom field using the search box. You can enter "<custom field>:<pattern to search>" in the search box and it will return you only the issues that have the custom field patching the pattern set.

@pingou in which section of document should I put docs for issue search tokenization.

We need to create a new document for this, likely improving the work started in https://pagure.io/pagure/pull-request/1944

Commit 52df0cc fixes this issue