From assembly lines to executive suites, thousands of blacks, women and other victims of job discrimination have been receiving promotions and millions of dollars in back pay under a court-enforced relaxation of seniority systems that once held them in low-pay, dead-end jobs.

Now the future of these plans - including the Bell System's massive job-ladder restructuring that put women in top jobs and a male voice on the phone when you dial for an operator - is in doubt.

The reason is a Supreme Court ruling last May that seniority systems that perpetuate the effects of past race or sex bias in hiring and promotion are not illegal unless the systems themselves were set up for discriminatory purposes.

This reversed a series of more than 30 lower appellate court decisions that were based on job discrimination, including discrimination that occurred before the Civil Rights Act took effect in July, 1965.

This previous interpretation of the law spawned a multitude of formal and informal agreements to bypass seniority rules so that blacks, women, Hispanics and other victims of previous discrimination would be given more access to better jobs.

Two of the biggest settlements involved the telephone and steel industries. Initial reactions of unions in the two industries indicate a mixed and widely varied pattern of response.

In the case of the telephone industry, the American Telephone and Telegraph Co. agreed in 1973 to a new pay and promotion plan that has led to a substantial expansion of better-paying jobs for minorities and women, who got more than two-thirds of the 61,000 new jobs and promotions offered by the company last year.

But the Communications Workers of America and other AT&T unions were not parties to the agreement and have been fighting it in court, withoug success, ever since. Union officials say the recent Supreme Court ruling has strengthened their case, which is now before the court in the form of a request for review.

In the case of steel, nine major steel companies and the United Steel-workers agreed in 1974 to overhaul their seniority system so workers would not lose pay or promotional advantages in moving from job to job.

The union's executive board said shortly after the decision it was not interested in taking advantage of the ruling to upset the agreement.

Taking a position somewhere in between the telephone and steel unions, the United Paperworkers is considering a policy of not attempting to reopen agreements made before the court's ruling but opposing future ones, especially those that involve back pay to aggrieved workers, according to one of the union's attorneys.

Seniority systems are hallowed concepts to labor unions because they establish the pecking order for existing workers on matters such as fringe benefits, promotions and protection against layoffs.

But they are flash points to civil rights and women's rights groups, which contend that - especially to the extent that they were built upon discriminatory work patterns - they lock many workers into inferior jobs.

A major problem is that many seniority systems are set up separately for each department within a plant. Thus, for instance, if a black worker in a dead-end job seeks to transfer to a better line of work with more advancement opportunities, he or she is forced to give up accumulated seniority from the old job and start over again in the new job.

For seniority systems established before 1965, the court's ruling appears to sanction such arrangements so long as the seniority systems themselves were not set up with a view toward perpetuating special advantages for whites or males. It also appears to make it more difficult for workers to attack systems adopted since 1965.

Lawyers, however, disagree sharply over precise interpretations - meaning more litigation.

Abner Sibal, general counsel for the Equal Employment Opportunity Commission, the government agency in charge of fighting job discrimination, contends that the decision does not jeopardize existing agreements and will have only a narrow impact on future cases.

"It may be much less significant than many people think," said Sibal.

A union lawyer agreed that the legal picture is murky, pointing to a footnote in the court decision that csats doubt on seniority systems negotiated by segregated union locals - a loophole that could be important in the South.

The major importance of the decision, the lawyer said, asking that he not be quoted by name, may be than the court appeared to free unions of responsibility for back pay awards in discrimination cases by legitimizing the unions' seniority systems.

Such awards in the past have been large. The steel agreement resulted in $30.9 million back pay, shared by the union and the nine companies. AT&T has awarded $11 million in back pay under its agreement.

A major question is whether the court's ruling will rpompt a rash of reverse-discrimination cases from white male workers who charge they were harmed by agreements that gave preference to minorities and women.

Several union officials and attorneys said they have heard "rumblings" of such challenges.

The steel agreement may be levulnerable to such attacks because loosening of seniority restrictions allow workers to carry their seniorit rights from job to job applied to a workers, even though minorities an women benefited the most, according to Elliot Bredhoff, special counsel for the Steelworkers union.

But Bredhoff and others agree that the ruling is likely to lessen not end the pressure for future agreements along this line.

"It undoubtedly puts a damper of them," said Bredhoff, although he noted that relief is still provided for discrimination that occurred after 1965.

Herbert Hill, national labor director for the NAACP, was more acerbic. At serting that the court ruling in the seniority and related cases signifie "the end of the era that began will the court's decision in Brown v. Board of Education in 1954," he said:

"The recent decisions give discrimnatory employers and labor unions new legal weapon to wipe out the gains of the past, to turn the clock back, and to more firmly impose racist patterns which continue to lock black workers in the lowest paying dirtiest and most unhealthy jobs where they are more vulnerable to unemployment than any other group in the labor force."

But at the same time, Hill, noting that the court's decision turned on an interpretation of the 1964 law, said seniortiy cases will continue to be pursued under constitutional requirements for equal treatment and Recon struction-era civil rights laws.

The Labor Department also take the position that anti-discrimination agreements with government contractors are not covered by the ruling because they stem from executive orders, not the 1964 law.