This paragraph just about sums up the entire 9-0 opinion per Justice Alito:
The factors that justify absolute immunity for trialwitnesses apply with equal force to grand jury witnesses.In both contexts, a witness’ fear of retaliatory litigationmay deprive the tribunal of critical evidence. And in neither context is the deterrent of potential civil liability needed to prevent perjurious testimony. In Briscoe, the Court concluded that the possibility of civil liability wasnot needed to deter false testimony at trial because othersanctions—chiefly prosecution for perjury—provided a sufficient deterrent. Id., at 342. Since perjury before agrand jury, like perjury at trial, is a serious criminaloffense, see, e.g., 18 U. S. C. §1623(a), there is no reason to think that this deterrent is any less effective in preventingfalse grand jury testimony
And Alito goes all historical to explain why “complaining witnesses” can be liable under 1983, but not for testifying before Grand Jury:
is true that a mid-19th century complaining witness might testify, either before a grand jury or at trial. But testifying was not a necessary characteristic of a “complaining witness.” See M. Newell, Malicious Prosecution 368 (1892). Nor have we been presented with evidence that witnesses who did no more than testify before a grand jury were regarded as complaining witnesses and weresuccessfully sued for malicious prosecution. See Tr. of Oral Arg. 14–15, 24–25.
In sum, testifying, whether before a grand jury or attrial, was not the distinctive function performed by a complaining witness. It is clear—and petitioner does notcontend otherwise—that a complaining witness cannotbe held liable for perjurious trial testimony. Briscoe, 460 U. S., at 326. And there is no more reason why a complaining witness should be subject to liability for testi- mony before a grand jury.
Oh that pesky Roberts Court closing the courtroom doors to civil rights litigation! These contentious 9-0 opinions really shake the public’s opinion in the rule of law.