Bitcoin Improvement Proposals are specifications that describe a new concept, protocol feature, or standard that its authors recommend for adoption to the Bitcoin community. The topic must be relevant to multiple software projects.

Usually, mempool policy defaults are selected by projects without coordination, unless they are picked coordinate interaction with other projects (see e.g. BIP 431). There are multiple node implementations that differ in mempool policy in a number of different ways. As such, a mempool policy change in Bitcoin Core might be discussed on the mailing list, but lower impact changes might even just be decided at the project level.

BTW, some BIP Editors have contended that mempool policy is generally not BIPable.