/etc/apache2/conf-available/matrix-synapse-plinth.conf is in plinth 0.24.0.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | ##
## On all sites, provide Matrix Synapse on a default path: /_matrix. This is
## only useful for clients to login without specifying a server port. This is
## not useful for federation which requires SRV record or listening on port
## 8448. Further, federation requires same TLS public key to be provided to
## Apache and Matrix Synapse server.
##
ProxyPass /_matrix http://localhost:8008/_matrix
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