Tax shelters were supposed to slip off their foundations when the maximum federal income tax rate was cut from 70 percent to 50 percent this year, but that hasn't happened.

Instead the tax bill passed by Congress has opened new avenues for avoiding the tax collector at a time when the Internal Revenue Service is slamming doors on shelters as fast as it can find them.

The basic idea of a tax shelter is to either produce income that is not taxable--such as the interest on some government bonds--or to create tax deductions that will offset income earned from other sources.

Some of the most exotic shelters have enabled investors not just to deduct all the money they invest, but three, four or five times that much as well.

Under the old law, even a one-for-one tax deduction meant that 70 percent of the money invested came out of Uncle Sam's pocket. Now the tax deduction provides no more than half the capital.

Cutting the maximum tax rate was supposed to reduce the appeal of shelters, but it may only produce a demand for shelters offering bigger deductions.

The notion that tax-free or tax-sheltered investments are only for the rich was put to rest when Congress created the All Savers certificates. Individuals don't have to pay taxes on the first $1,000 of interest income from the certificates.

Congress also allowed individuals covered by employer retirement plans to start an Individual Retirement Account after next Jan. 1 and deduct from their taxable income investments of up to $2,000 a year in the account.

Most of the best breaks in the 1981 tax bill went to business, but there were other sweeteners in the tax package for individual investors. Real estate investments in particular offer better opportunities for offsetting other income as a result of recent changes in law.

But the economics and cash flow of tax sheltered investments will become more critical than tax write-offs as a result of tax law changes, says Barry Goodman, director of financial planning for Leopold & Linowes, certified public accountants.

The immediate tax deductions for real estate investments will be bigger than they have been, and the threat of "tax poisoning" that results in higher future taxes has been removed, Goodman noted.

What the accountants call "poisoning" occurred under the old tax law when real estate investors chose to claim accelerated depreciation--and therefore bigger tax deductions--on property; once the quick write-off had been taken, investors faced higher taxes on future profits from the sale of the property.

The new tax bill reduces the depreciation period for most real estate from 40 years to 15 years. The option of claiming special accelerated deprection was eliminated and, along with it, the provisions that allowed the government to "recapture" profits.

Now all real estate investments qualify for the quick 15-year write-off, which provides bigger tax deductions than did the accelerated depreciation option of the old law. Real estate investors come out ahead.

The new law is producing no boom in real estate shelters, however, because of today's high interest rates. The real estate market is already soft and if it gets worse, tax shelter investors could take a beating, cautions Goodman.

Another warning comes from Internal Revenue Service Commissioner Roscoe Egger, who vows to end what the IRS calls "abusive tax shelters."

Tax shelters the IRS considers "abusive" are those involving artificial transactions with no purpose but to create tax deductions, inflated appraisals of the value of deductions, unrealistic allocation of expenses in a project, or what Egger calls "schemes lacking economic reality."

In the midst of last spring's congressional tax-cutting fervor, tax shelter opponents managed to outlaw one of the most outrageous ways of creating tax deductions, a commodity investment technique known as a "tax straddle."

The straddle involves buying and selling commodity futures contracts simultaneously, knowing one of the transactions will lead to a profit and the other will result in a loss. The loss is deducted from the current year's taxes, while the profit is not reported until the next year. By repeating the process annually, some investors avoided taxes year after year.

In the future, investors will no longer be able to generate artificial tax losses from commodity straddles. Previous users of tax straddles will also face the loss of tax deductions they have taken, if the IRS wins a case that is expected to be decided by the Tax Court before the end of the year.

Stock options straddles are the next target of the tax collectors. Options trading techniques to avoid or delay taxes may be a gray area now, but the IRS is moving quickly to make them clearly illegal.

Other tax shelters likely to draw auditors' attention include investments in lithographic plates, deductions for donating Bibles to charities and various investment schemes involving movies, coal mines and foreign trusts.

As one IRS specialist put it: "If a tax shelter scheme sounds too good to be true, it probably is."

In other efforts to end abusive shelters, a tax shelter task force has been set up at the IRS, three new judges have been added to the federal tax court to speed up trials in tax shelter cases, and the penalties for trying to cheat the tax collectors have been toughened.

In a speech last month to tax specialists, Egger said the revenue service is also cracking down on lawyers who write legal opinions endorsing dubious tax shelters and is preparing legislation to expedite audits of shelters.

The IRS has already changed its audit proceedures to target returns that show large deductions and adjustments to income. It has also changed its audit proceedures to single out taxpayers who utilize shelters to reduce their tax liability.

The IRS used to look primarily at the amount of adjusted gross income to determine whether a tax return was worth auditing. A taxpayer with a $10,000 adjusted gross had little chance of being audited. But a $190,000 loss on a tax sheltered investment can reduce a $200,000-a-year professional's salary to an adjusted gross of $10,000. So IRS now looks at what it calls Total Positive Income and picks returns to audit on that basis, before deductions or adjustments.

When an audit turns up taxes that are due, taxpayers this year will have to pay 20 percent interest on the overdue amount. Before recent changes in the tax law, the interest rate on back taxes was 90 percent of the prime rate, adjusted every two years. That rate amounted to 12 percent last year, so low that it was advantageous to stall the IRS for as long as possible,taking what amounted to a low-interest loan from the government. Now the interest is at the prime rate, adjusted annually.

If an audit determines additional taxes are due because tax shelter deductions were overvalued, the penalties will be stiffer. In the past, the only penalty was the 12 percent interest charge, which itself was tax deductible. The new penalty is a 50 percent to 60 percent additional tax, no longer deductible.