Housing supplies increase when developers build new housing. Landlords would seem to be a fairly small component in that equation.
The supply didn't increase. Demand reduced, because you moved away.
By your logic, if I stop eating hamburgers I've increased the supply of hamburgers.
Even in the 'normal' development sector, landlords buy a significant proportion of the apartments available from new build developments. I would estimate that 30-40% of the apartments in my building were bought by landlords ahead of the development starting. This adds liquidity to the market and allows developers to build knowing they are taking less risk on selling them when the building is (more) complete.
That is very similar to this quote of Smith:
> The rent of land, therefore, considered as the price paid for the use of the land, is naturally a monopoly price. It is not at all proportioned to what the landlord may have laid out upon the improvement of the land, or to what he can afford to take; but to what the farmer can afford to give. (Adam Smith, op. cit., p. 131)
> Rent, considered as the price paid for the use of land, is naturally the highest which the tenant can afford to pay in the actual circumstances of the land. (Adam Smith, op. cit., Vol. I, p. 130)