It's not the whole picture, but it's far from useless. There are a myriad of different unemployment metrics. Some of them include people who have given up looking. However, all of these metrics are highly correlated. It's very rare for one to move without the others moving proportionally.

The labor force participation rate being down is just an indication that the population is aging and people are retiring. If you look at the prime age labor force participation rate, it's at the highest point in more than 20 years and very close to an all time high[1].

[1] https://fred.stlouisfed.org/series/LNS11300060