The percentage of adults in the U.S. without a high school degree has declined by about 67% since 2020 — from 1990 to 2019 an absolute decline in persons 25 years or older by 18 million persons even while the total 25 year of older population grew by 67 million (147 to 209).
The majority of persons working in the U.S. today without a high school degree are foreign born, even though the foreign born make up only 14% of the total population. This dramatically shows how this cohort of workers is concentrated among immigrants.
In 2020, the labor force participation rate of foreign born vs U.S. born of persons 25 or older is higher among foreign born for all levels of formal education, and sharply so for those without a high school degree and with a high school degree. (Go here.)
Without a high school degree, the participation rate for foreign born is 56% (5 million persons working ) vs 35% for U.S. born (4 million persons working).
In 1990, there were 42 million persons 25 years-plus who did not have a high school degree in the U.S., or 30% of the total population 25 years-plus. By 2000, this number had dropped to 31 million, and by 2010, it had declined further to 24 million. The most recent data available from NCES is from 2019, which shows that there were 20 million persons 25 years-plus who did not have a high school degree in the U.S, or 10%.
These foreign-born workers without a high school degree are concentrated in construction such as painters, drywall installers, and construction laborers; food service and hospitality; agriculture, including picking crops and performing other farm labor; cleaning and maintenance such as janitors, house cleaners, and building cleaners; an labor-intensive manufacturing industries such as textiles, furniture, and plastics. I think that the majority of these workers are unauthorized, and many with limited English proficiency, both of which keep them out of many jobs.