Waaaayyyy old info. Does not apply to current version (4.28) July 2020. And there should be one simple button to do it. Currently you have to change several settings for the thread. Ridiculous that the best texting app lacks this feature.

If I understand what you're asking for, they already have this. Go to the main screen as if you're doing a batch delete of multiple senders' threads. Select the same way (can select just one). Go up to top right corner. Click the dots. Select the per contact option. Once there, select the notifications option. Turn off notifications, change sound, flash color, icon color etc there. :-)

Please!