Which Popular Cryptocurrency Projects Have Utilized or Proposed a Proof-of-Activity Model?

While not as widely adopted as pure PoW or PoS, the Proof-of-Activity model was notably proposed and utilized by the Decred (DCR) cryptocurrency. Decred uses a hybrid system called Proof-of-Work/Proof-of-Stake.

This system mandates that a majority of stakers must approve all blocks mined by PoW miners. This implementation enforces governance and prevents miners from dominating the network.