SD Express is basically dead as far as I can tell.
SD card users who care about speed have UHS-II equipment, but SD Express and UHS-II use mutually incompatible high speed signalling on the same pins (so cards and readers are only supporting one of the two - I guess technically this could be fixable with special purpose chips, but at large cost).
Users who care about speed but not about SD card compatibility are already using CFExpress, which is supported by most modern professional cameras and has much better hardware availability than SD Express.
SD card users who care about speed have UHS-II equipment, but SD Express and UHS-II use mutually incompatible high speed signalling on the same pins (so cards and readers are only supporting one of the two - I guess technically this could be fixable with special purpose chips, but at large cost).
Users who care about speed but not about SD card compatibility are already using CFExpress, which is supported by most modern professional cameras and has much better hardware availability than SD Express.