Regarding the post title: Lemmy itself allows you to block communities, instances, and users. But it does not support filtering by keyword, e.g. filtering out any post with the word “platypus” in the title regardless of where it was posted or by whom. With help from the comments below, here are some third party Lemmy clients that have their own implementation of keyword filtering:

• Boost
• Connect
• Eternity
• Interstellar
• Raccoon
• Summit
• Sync
• Tesseract
• Thunder
• Voyager

Do NOT support keyword filtering:

• Alexandrite
• Jerboa
• Lemmy UI
• mlmym
• Next
• Photon
• Quiblr

Comment anything I have missed or mistakes I made