As Rihanna has now been confirmed as pregnant with her second child, according to her team, your attention to her outfit makes sense - with this being said, her top was not an anti-lactation article of clothing.
Rihanna's outfit at her 2023 Super Bowl performance has many layers and significance, though. The outfit was brought to us by Lowe, a luxury Spanish brand that specializes in leather wears. Lowe is a brand under the fashion house conglomerate of Moët Hennessy Louis Vuitton, commonly known as LVMH. On their website now, they have an piece highlighting Rihanna's outfit and its influences. As her leather top was made by Loewe, it is not marketed as a top to prevent lactation. The outfit was inspired by aviation suits, with a sleek and dynamic twist to match her performance, as per Loewe's description of their styling.
While the top may or may not have suppressed Rihanna's lactation, this was not the intended purpose of the outfit; maybe a mere bonus for her to wear while pregnant and performing.
Rihanna's Representative Confirmation: https://www.nytimes.com/2023/02/12/sports/football/rihanna-pregnant-super-bowl-.html
Loewe Article: https://www.loewe.com/usa/en/stories-collection/Rihanna-superbowl-custom-look.html