I’m curious if there are any projects in development that could supplant anonymous remailers, as they’re the only option if one wishes to completely mitigate timing-based attacks when sending messages. The problem is that all are email-based, which, in my opinion, negates any privacy-preserving aspects of these services. Remailers are good for sending email anonymously, there is no reason to use them otherwise.
That being said, for more extreme threat models where metadata deanonymization is a concern, no protocols or services currently exist that mix traffic and delay timing for two-way communication. This is a highly niche topic but I feel it’s worth exploring for privacy advocates with continually expanding threat models.