In JSP for my Liferay Portal plugin I use specific TLDs:

<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@taglib uri="http://java.sun.com/portlet_2_0" prefix="portlet" %>
<%@taglib uri="http://liferay.com/tld/portlet" prefix="liferay-portlet" %>
<%@taglib uri="http://liferay.com/tld/theme" prefix="liferay-theme" %>

However, the second is underlined with the hint:
The absolute uri: http://java.sun.com/portlet_2_0 cannot be resolved in either web.xml or the jar files deployed with this application

It is strange as that TLD is referenced in proper loaded library:
<dependency>
    <groupId>com.liferay.portal</groupId>
    <artifactId>util-taglib</artifactId>
    <version>6.2.1</version>
    <scope>provided</scope>
</dependency>

It looks like something has changed in TLD resolving as it works fine in 8.1.