I had successfully used Silent Boot app to disable Tmobile startup jingle on an S8. When that app was opened on my new (open box) S20 (with 5G and android 11 upgrade) a notification appeared saying it was an older version not intended for S20.
I opened it up and easily set it up to mute device on shutdown. My first restart after that played about 1 seconds of the jingle and subsequent boots have been silent.
Wish I had tried this first - rather than spending about an hour fruitlessly searching the web.
I'll provide an update if it fails to continue to work well.