IT LOOKS LIKE thin times ahead for dancers, historians, opera companies and repertory theaters. The Reagan administration is now severely pruning back the federal subsidies to culture that have grown up since the middle 1960s, but that's only part of the financial squeeze ahead of them. The Presidential Task Force on the Arts and Humanities published its report yesterday, calling on Mr. Reagan to help raise private money to replace the diminished subsidies. But, tempering hope with the realism of experienced fund-raisers, the task force found it difficult to believe that additional private philanthropy will fully compensate for the reductions in federal funds. The gigantic tax bill that Congress enacted last summer sharply shrinks the tax advantages that have traditionally encouraged philanthropy.

The task force was set up last spring in response to the artistic and humanitarian outcry over the first stinging application of Stockmanism to the subsidies. The task force was asked, essentially, whether any federal subsidies were justified. After all, defining quality in the arts and scholarship is not a thing that governments generally do well. Some of the money was patently going into cultural pork-barreling, with grants designed to reach the largest possible number of congressional districts. Should the federal government be in these fields at all?

The task force represented a great diversity of political opinions, and one of its most useful achievements was to provide a unanimous answer to that question. The diversion of money into frivolous ventures, and arts with no audiences, can be held down by requiring substantial matching by private contributions. Review by experts who do not work for the government offers insulation against political meddling. Beyond that, you have to conclude that a certain risk of occasional waste is worth taking. Without any federal help, some institutions of the highest quality and genuine national importance--the Metropolitan Opera is at the top of the list--would be unlikely to survive.

The task force and its report probably ensure that the National Endowments for the Arts and the Humanities will stay in business through the Reagan years, although not at pre-Reagan levels of affluence. But even at their peak, the $300 million that the two endowments together spent annually was perhaps one-tenth of the total that Americans contribute, one way or another, to cultural activities. Most of the federal contribution was not in the form of direct funding from the endowments, but in the deductions from private donors' taxable income.

By drastically lowering the tax rates for the people with the most to give, this year's tax law sharply decreases the tax incentives for gifts to universities, orchestras and the rest. Many of the task force's recommendations anxiously address the tax laws, and urge a variety of new tax inducements to replace the vanished ones. That's not a very promising idea. Federal revenues are now so low that it is impossible to support further tax credits and deductions even for the best of causes. The primary responsibility for supporting American cultural life again lies, as it did long ago, with the disinterested private donor.