Ideally, a full log of joins and leaves wouldn't be bound directly to a chat anyway, right?
Matrix development seems to be getting pulled in a lot of different directions: the membership history seems like it's an IRC-inspired feature, while communities are inspired by Discord and Slack.
Brave and Firefox are very competitive when it comes to pushing unnecessary "features" on their users. (Remember when Mozilla bought an NFT and AI company to put a shopping toolbar in their browser?)