How do I detect if there are any mobs in a specific area and execute a command when they are no longer present in that area?

If you really wanted to you could just do a really long command that checks for each potential mob type in the floor.

execute unless entity @e[type=zombie,x=14,y=12,z=21,dx=20,dy=8,dz=13] unless entity @e[type=creeper,x=14,y=12,z=21,dx=20,dy=8,dz=13] unless entity @e[type=spider,x=14,y=12,z=21,dx=20,dy=8,dz=13] unless entity @e[type=skeleton,x=14,y=12,z=21,dx=20,dy=8,dz=13] unless entity @e[type=enderman,x=14,y=12,z=21,dx=20,dy=8,dz=13] unless entity @e[type=silverfish,x=14,y=12,z=21,dx=20,dy=8,dz=13] unless entity @e[type=endermite,x=14,y=12,z=21,dx=20,dy=8,dz=13] run setblock 30 14 26 acacia_button[facing=south]