The Rams did wear their throwback uniforms in SB53 but I think the NFL considered the Rams situation to be unique.
The 'situation' being the Rams desire not to wear the 'St. Louis Rams' uniforms in LA. They were allowed to wear their throwbacks 5 times in the 2018 regular season (instead of three times, which is normally the limit) and then in SB53. But I'd bet good money that the NFL would say 'no' to alternate jerseys this year. And I seriously doubt that the Rams would be interested in doing that anyway.
AlbaNY_Ram