dnd 5e – Does the Swords bard’s Slashing Flourish work with the Swashbuckler rogue’s Fancy Footwork to prevent opportunity attacks by any affected target?

This does not work; the extra damage from Slashing Flourish is not an “attack”.

The Swashbuckler rogue’s Fancy Footwork feature says (XGtE p. 47, SCAG p. 135; emphasis mine):

(…) During your turn, if you make a melee attack against a creature, that creature can’t make opportunity attacks against you for the rest of your turn.

The Swords bard’s Slashing Flourish option for the Blade Flourish feature says (XGtE, p. 15; emphasis mine):

You can expend one use of your Bardic Inspiration to cause the weapon to deal extra damage to the target you hit and to any other creature of your choice that you can see within 5 feet of you. (…)

Finally, the rules for “Making an Attack” say (emphasis mine):

If there’s ever any question whether something you’re doing counts as an attack, the rule is simple: if you’re making an attack roll, you’re making an attack.

Since the extra damage to another creature does not involve an attack roll, and the feature doesn’t say you’re “making an attack” against those other creatures, you are not “making an attack” against them. However, Fancy Footwork requires that you make an attack against the creature. Therefore, Fancy Footwork would not affect them; it would only prevent opportunity attacks from the initial creature that you actually made the attack roll against.