The stolen base might be a dying strategy, but just because totals in that particular category were down 5% in 2020 compared to five seasons before it doesn't mean that speed is any less important a factor in today's game.
For us in fantasy, speed manifests in, most obviously, the stolen base. It's one of the five traditional rotisserie categories, and therefore we need to accurately project them in order for our teams to succeed. Speed -- and it's italicized in the lede for this reason -- has other influences, however, including its impact upon defense (quality defense improving a player's chance at additional playing time and, therefore, more chances to contribute to our teams), on runs scored (baserunning beyond the stolen base itself) and -- this one you might underrate -- on batting average.
As with any facet of today's game, there are even advanced metrics pertaining to a player's speed. Among those that I find more telling is Statcast's sprint speed, which measures a player's raw speed in, as they describe it, "feet per second in a player's fastest one-second window." The league's average is typically 27 feet per second, with most players falling within a range of 23-30 feet per second. Statcast also tallies Bolts, which count the number of times a player exceeded a 30-feet-per-second sprint speed.
Sprint speed alone, however, isn't a driving force behind raw stolen base totals. A player's ability to read pitchers' motions, the jumps he gets, the pitcher-catcher matchups he chooses to exploit and, perhaps most importantly, the green lights that his manager gives him all enter into the equation.