Bitcoin proposal BIP-420 proposes to restore OP_CAT opcodes

Bitcoin Improvement Proposal BIP-420 officially proposes to restore OP_CAT opcodes to introduce "constraints" (Covenants) functionality on Bitcoin. OP_CAT advocate Udi Wertheimer, co-founder of Taproot Wizards, said that OP_CAT will allow smart contracts, secure cross-chain bridges, on-chain transactions, zero-knowledge proof verification and other functions on Bitcoin.
The so-called "constraint" is an advanced scripting feature that can set specific conditions for the use of bitcoin in future transactions. This will open up a variety of application scenarios, such as creating secure vaults that allow transactions to be rolled back, automating recurring payments, setting timed transfers for scenarios such as inheritance, and complex financial instruments such as escrow and bonds.
The proposal, written by Ethan Heilman and Armin Sabouri, aims to restore OP_CAT by redefining opcode OP_SUCCESS126 in a backward-compatible way. However, OP_CAT soft fork can be achieved depends on multiple considerations such as technology, security and community consensus.

The concept of Bitcoin "constraint" can be traced back to at least 2013. In addition to OP_CAT, there are several "constraint" proposals such as CTV, CSFS, LNHANCE, etc., which are currently in different stages of research and discussion.