Here is the clear answer:

Olivia’s mother, Maya Pope (born Marie Wallace) , and father, Rowan Pope (Eli Pope) , were never legally married to each other. They had a romantic relationship while both were covert operatives, but no divorce occurs between them. Rowan is later revealed to have killed Maya (before she returns alive in Season 3), but again, no marriage or divorce.

While the official divorce occurs in Season 5, the relationship was "dead" long before that. Varietyhttps://variety.com

At the start of Season 5, Fitz and Olivia Pope's affair is made public, making his presidency vulnerable to impeachment.

Reviewers and fans often view the divorce as a turning point for both characters, though opinions on the execution are mixed:

While the couple had a strained marriage for decades due to Fitz’s affair with Olivia Pope, the true catalyst for the divorce occurred late in Season 4.

Fitz serves Mellie with divorce papers during his impeachment hearing. To avoid testifying and potentially perjuring herself or hurting Fitz, Mellie signs the papers, officially ending their marriage. Critical Reviews and Perspectives