Listener Thread: running in debug log

Yes

Starting 11.17.x version we see a string "Listener Thread: running" introduced in the debug log

0:10:23 (demo) Listener Thread: running
 0:10:23 (demo) (@demo-SLOG@) ===============================================
 0:10:23 (demo) (@demo-SLOG@) === Last 10 Client Requests Processing Time (in ms) ===
 0:10:23 (demo) (@demo-SLOG@) Time: Sat Aug 28 2021 00:10:23 PDT
 0:10:23 (demo) (@demo-SLOG@) Request processing time, when, #concurrent clients, client info (user, node, FD)
 0:10:23 (demo) (@demo-SLOG@) 00000000 ms at Sat Aug 28 2021 00:10:23 PDT,#3,(ansl,liro-r981.demo.com,28)

"Listener Thread: running" has been introduced as a new thread for connection, it is not related to Diagnostics feature in anyway.
The purpose of it is to accept new connection coming to license server and to improve server performance.
It is done for optimization purpose.