Several Bitcoin Improvement Proposals (BIPs) have been proposed but never activated, often due to lack of consensus, security concerns, or changes in Bitcoin’s development priorities. Here are some notable ones:

1. BIP 101 - Increase Block Size to 8MB (2015)

• Proposed by Gavin Andresen, this BIP suggested increasing the block size limit from 1MB to 8MB, doubling every two years until reaching 8GB.

• It was controversial and never activated due to fears of centralization and network strain.

2. BIP 102 - Temporary 2MB Block Size Increase (2015)

• Another block size increase proposal, this time to 2MB as a temporary measure.

• It was seen as a compromise but never gained enough support.

3. BIP 109 - 2MB Block Size via Miner Signaling (2016)

• Proposed by Bitcoin Classic, it suggested a 2MB block size increase if 75% of miners signaled support.

• It failed to reach activation thresholds and was ultimately abandoned.

4. BIP 148 - User-Activated Soft Fork (UASF) for SegWit (2017)

• This BIP aimed to force Segregated Witness (SegWit) activation by rejecting non-SegWit blocks after a certain date.

• While BIP 148 itself did not activate, its pressure led to miners adopting BIP 91, which enabled SegWit activation in a different way.

5. BIP 75 - Payment Identity & Interoperability (2016)

• Proposed a standardized way for wallets to exchange identity and payment information.

• It was not widely adopted due to privacy concerns.

6. BIP 300 & BIP 301 - Drivechains (2017, Updated 2023)

• Introduced a way to implement “sidechains” using a new security model.

• Controversial due to security risks, and has yet to gain widespread consensus.

7. BIP 50 - Chain Fork Post-Mortem (2013)

• Not a proposal for activation, but rather a documentation of a major chain fork in Bitcoin’s history.

• Highlights issues that could arise in future soft forks.

These BIPs reflect Bitcoin’s ongoing debate over scalability, security, and decentralization.