Many journalism organizations,over the last three decades,have been working on glossaries to deal with loaded terms...last three decades is time mentioned then why is it in present perfect tense
A mention of a duration of time is acceptable with present perfect forms, with words like "for" or "over":
"They have been working there for/over the last three decades."
The rule that we should not mention time with present perfect forms applies to time-related words and phrases that indicate when the event started/occurred. The exception is when you use the word "since".
"They have been working there in 1982." - Wrong
"They have been working there since 1982." - Correct