Is it safe to use electrical sockets not put into the wall as awaiting plaster to dry

No it’s not habitable, it’s highly dangerous. That’s the reason we have socket outlets secured to a box. Agree with the above comment about the landlord being a chancer. Who unscrewed them in the first place?

The short answer is ‘Don’t do it!’ I would urge you not to take any chances. If the sockets are not secured to the wall then it is possible that the wires at the back are exposed and could pose a risk of electric shock to you or your family. Similarly they may be vulnerable to things being spilled as just one of the first examples that come to mind. There is a reason why sockets are secured and your landlord should not encourage you to inhabit a dwelling where the electrics are unfinished. I am sure that it wouldn’t pass any type of safety inspection as it is. If you feel that you are being put under pressure then might I suggest that you tell your landlord that you do not have a sufficient level of expertise in this field to decide and that you will be happy to accept the opinion of a local authority housing inspector. I’m sure your landlord knows that it’s not right, he’s just being a bit of a chancer. Safety first.

To not fix the sockets back would be a C1 departure (Danger present - immediate remedial action required)

It's definitely unsafe! It is also illegal. Having sockets hanging out of the wall would immediately fail EICR which landlords are obliged to have to ensure electrical installation is safe for tenants.