There's a Reddit post noting that only 5( really 6) MLS teams have failed to win some hardware since joining MLS: The oldest - Minnesota - just joined MLS in 2017. The others are Cincinnati (2019), Miami and Nashville (both 2020), and Austin 2021. They left out Charlotte but if you count them even though they haven't had a full year, they make 6. Cincinnati and Minnesota have USL or NASL hardware.
Even more amazing IMO, a comment lists the most recent domestic (i.e. non CCL) trophy and year for every MLS team. The longest no trophy streak is Chicago who last won USOC in 2006. But then, after just 5 more teams, it jumps to 2015 and Portland winning MLS cup in NYCFC's first year. Since and including 2015 Portland, 16 different MLS teams have won MLS Cup, the Shield, USOC, or Canadian Cup (Voyageurs) in 8 years.