It’s worth checking if your landlord didn’t give your deposit to one of three protection company deposit schemes. These can be on the basis of insuring the money in an insurance based policy with that scheme or a custodial depositary type scheme.
If a landlord doesn’t protect it the housing act allows tenants to make a claim for x3 the deposit presumably as a punitive measure to ensuring the money is protected if the landlord or agent goes out of business and the tenant looses the money.
I just found out my landlord didn’t protect my deposit around £1k for some time after the start of the tenancy, so I will make a claim after I’ve left. It also invalidates my section 21 so it’s going to buy me time in leaving my current property, if need be.
I would check the timings as to when you did become aware. The statute of limitations will impose timescales on when cases can be brought but if the landlord didn’t tell you they had protected the deposit until when you got the section 21 notice and presuming your case wouldn’t be statute / time barred, then you can argue you didn’t have ‘constructive knowledge’ of the protection scheme until you got the notice and obviously it’s on landlords or agents to tell you of where the money is protected once they protect it.