The situation, and its implied status, has definitely changed just recently. It doesn't seem as though it "just continues to be unused".
Since they failed to reopen the entrance post-COVID, it's been "as was" inside (as far as one could tell from looking through the glass!) and presumably could have been reopened to provide additional access to the tube again (it was great for the Victoria Line, though less so for anything else since they changed the interconnecting tunnels there as part of the Kings Cross upgrading some years back). It was very useful for people starting or finishing a tube journey at any of the places a block or two to the east of the main Kings Cross, to save an extra walk and to save having to use a more congested route. However, there was the matter of the escalator which needed fixing (and the stairs were perhaps a deterrent for people these days!) - which had been the case for some time before the station closed; so perhaps this was part of the reason for not reopening it.
It has also been seen in the past as a useful escape route - being well away from the other exits from the station - in case of emergency.
It's certainly true that it obviously got less use once the Thameslink station was moved, but did have a steady usage in the years after - especially at peak times (working nearby, I often used it). It's also true, however, that since the patterns of tube use haven't returned to the pre-COVID situation, then operating the extra (and least-used) KX tube entrance would likely be judged as not worth the cost at the moment.
I always assumed that as tube use grew back, it would eventually be seen as worth re-opening it (including for its emergency escape role) again. But just in recent weeks the whole frontage has been boarded up along the edge of the pavement, as though there might be some serious demolition going on inside at some stage. (It's so sealed off that I doubt there'd be any way at all now for its use as an emergency escape from either the tube or the Thameslink tracks via the old platforms.) Given that the building runs alongside the tracks there, and since some of the non-public space could perhaps include railway-related equipment, I'd be surprised if a decision were made to demolish it and build something completely new there.
But clearly the situation is now something other than the "leave it unused for a while until we need it again" situation of the last couple of years
I assume someone must know more...!