The long-term decline in the labor force participation rate is largely driven by our aging population. More than 10,000 people in this country turn 65 every day, and at least some of them are leaving the workforce.

A much more relevant metric is the labor force participation rate for prime-age workers (ages 25-54): https://fred.stlouisfed.org/series/LNS11300060

That metric has been rising since 2015 and is back near pre-Great Recession levels.

Also, a more comprehensive unemployment rate metric is the U-6 rate, which includes all persons marginally attached to the labor force (including discouraged workers), plus those employed part-time for economic reasons: https://fred.stlouisfed.org/series/U6RATE

That metric is near 25-year lows.

So the labor market is strong by these more relevant metrics - for now, at least.