Idioms related to uninteresting topics

Solution 1:

Yesterday's news,

from merriam webster

something or someone that was popular once, but is not popular now

That actor is yesterday's news.

and

Yesterday's news from Oxford online dictionaries (scroll to bottom of link )

A person or thing that is no longer of interest.

‘Now we hear that he has been charged with adultery and having pornographic material in his possession; the espionage accusations are apparently yesterday's news.’

BSE is not yesterday's news and anyone who relies on governments to guarantee the safety of what they eat simply hasn't been paying attention…’

Solution 2:

old hat.

Used to refer to something considered uninteresting, predictable, tritely familiar, or old-fashioned. (definition reference: Dictionary.com)

Not really used to describe people.