It's an "interstitial" filter plate. It performs exactly the same function as a filtered faceplate or even a dangly filter.
It is used by Openreach these days as it plugs into the test socket, then provides a filtered phone socket in front of that, for the faceplate to plug into. It's very quick to fit, as the extension sockets that are wired to the back of the faceplate stay so wired, but are filtered because the supply to the faceplate is.
If they fitted a filtered faceplate they would need to disconnect the extension(s) from the old one and reconnect them to the new.
It appears the filtering in the new one is slightly better than earlier filters.
Apart from its warranty, (AIUI!) it is your property and responsibility to replace. I believe they are available to buy.
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