Buyback cannot be used until a certain duration after the Hero dies.
Buyback delay is based on the number of times the Hero has bought back previously. The maximum number of previously used buybacks that affect the delay is five. (NOTE: this could be as short as three or four if five seconds seems too long.)
Notes:
Separate from the buyback cooldown itself. This added delay is intended to make it easier to break high ground the longer the games goes. The longer game, the more levels/items/damage the defending team has, and that advantage has swung a tad too far in their direction. Picking off a defending Hero followed by the instant buyback/TP in/turnaround is all too common. The attacking team is hardly rewarded at all in the moment if the picked off Hero has buyback (and in late game, they almost always do because teams stall just to farm them up or wait out the timer). In the late/ultra late game, the more those precious extra seconds matter to the attacking team because damage output is high enough that buildings/Heroes/Roshan can die in mere seconds.
This change won't necessarily discourage buybacks earlier in the game. If a Hero must buy back to defend a barracks or contest Roshan, the extra second of delay adding on in the late game won't outweigh that decision in the moment if it's important enough.
This idea was inspired by Black King Bar's rework (from patch 6.55) when its cooldown but more importantly its duration was reduced to a certain limit based on previous uses.