Any time the government tries to encourage a certain behavior by making that behavior less expensive, it's a subsidy. Even if it's indirect.
Government wants people to buy houses? Homeowners get to deduct the mortgage interest from their income tax calculation.
Government wants people to go to college? People who open 529 plans don't have to pay taxes on the income generated in those accounts.
Those aren't direct subsidies. The government isn't cutting a check to the homeowners or the people paying tuition but, as explained by the
Global Subsidies Initiative:
In countries with well-developed tax systems, subsidies provided by reducing tax burdens are commonplace. Examples include tax exemptions (when a tax is not paid), tax credits (which reduce a tax otherwise due), tax deferrals (which delay the payment of a tax) and a host of other instruments. In common language these preferential tax treatments are called tax breaks or tax concessions; public-finance economists refer to them as tax expenditures. They should not, however, be confused with general tax reductions.
Generally, when a government provides a tax break its budget is affected in much the same way as if it had spent some of its own money.
it is still a subsidy.