Those condition classes extend com.atlassian.jira.plugin.webfragment.conditions.AbstractIssueWebCondition but it seems the class is not accessible by default in a plugin project.
I need to add this dependency to see the Condition class:
<!-- Plugin web fragment API, for the com.atlassian.plugin.web.Condition class -->
<dependency>
<groupId>com.atlassian.plugins</groupId>
<artifactId>atlassian-plugins-webfragment-api</artifactId>
<version>5.3.2</version>
<scope>provided</scope>
</dependency>
And the issue object is stored in jiraHelper.getContextParams(), under the key “issue”.