Synapse's logs (structured or otherwise). Not metrics.
Issues related to enforcing usage limits according to the number of monthly active users.
Uploading, downloading images and video, thumbnailing
Issues related to Synapse's support of MSC1763 (message retention policies)
/messages client API endpoint (`RoomMessageListRestServlet`) (which also triggers /backfill)
metrics, measures, stuff we put in Prometheus
Tools for moderating HSes: event redaction, media removal, purge admin API, reports from users, ...
Module API: https://matrix-org.github.io/synapse/latest/modules/index.html
Our Debian packages, docker images; or issues relevant to downstream packagers
Performance, both client-facing and admin-facing
Script for exporting sqlite data into postgres
Issues related to push/notifications
Retention rules to delete messages after a certain amount of time
Related to textual search of something, be it rooms, users or messages.
Login via external identity providers
Hierarchical organization of rooms
places where synapse does not conform to the spec