A good summary there from @JohnnyJ.

The guest plus floorspot format seems to be on the decline with a tend to move either to a full concert format or a more informal singaround. Having said that there are still plenty of clubs running the classic format successfully but the smaller clubs that I know have gradually been cutting back on booked guests.

Post CV19 we really don't know. Every club and concert series will need to relaunch as if it is totally new and there may be issues in scheduling reopening events timed to relaxation of restrictions if notice is short.

Apart from the risk that many venues will not reopen at all I suspect that the trend towards a split between regular singarounds and ad hoc concerts will increase.