Federal grand juries can and have indicted former President Donald Trump, as he no longer has the legal immunity that sitting presidents enjoy.
Federal Grand Juries can indict Trump

Trump indictment would be unprecedented in US history
Source: toronto.citynews.ca
Key Evidence
The Toronto CityNews article explains that sitting presidents cannot be federally indicted but Trump lost this shield after leaving office.
toronto.citynews.catoronto.citynews.caTrump indictment would be unprecedented in US history
Britannica details a 37-count federal indictment against Trump related to classified documents.
britannica.combritannica.comFederal indictment of Donald J. Trump | Full Transcript, Classified Documents,...
The Hill reports on a federal grand jury convening specifically in a case against Trump concerning the 2020 election and transfer of power.
thehill.comthehill.comGrand jury convenes in Trump case: Here’s how grand juries work
What the Evidence Shows
The longstanding Justice Department policy prohibits the federal indictment of a sitting president, but this protection does not apply once the president leaves office. Donald Trump, having been out of office for over two years, is subject to federal grand jury indictments. Multiple sources confirm that federal grand juries have convened and returned indictments against Trump, including a notable 37-count indictment related to classified documents found at his Mar-a-Lago estate.
This makes Trump the first former U.S. President to face such federal charges. While some federal grand juries in Washington, D.C.
Have recently declined to indict individuals accused in cases connected to Trump's law enforcement crackdowns, these instances do not contradict the fact that grand juries can indict Trump himself. The rare refusals to indict others reflect local grand jury decisions rather than any legal barrier to indicting Trump. Overall, the evidence clearly shows that federal grand juries have the authority and have exercised it to indict Donald Trump after his presidency.