It does get dimmed for me!

Same here.

FWIW, it always stays highlighted for me. Even when I have read and unfollowed the threads. I just assumed that it never would be dimmed. Now that I know that it does get dimmed for some, I'll go looking to see why it doesn't for me. If anyone finds out or has guesses, please report back. Thanks.

I am using Slack web interface via Google Chrome, in case that makes any difference.

I know there is a Slack app for various platforms, but have never tried it out.

Also using through chrome, standalone app is too heavy.

I am also using the Slack web interface on Google Chrome on a Mac.

Threads gets dimmed for me once I've caught up with new stuff. Edge (Chromium) on Mac and Windows. Maybe there's a preference in Slack for marking items as read that you have set differently?

You've got unread items that are older than the free slack message limit (10,000)

I think @UK7SDJD71 might be correct. I checked my preferences but did not find anything different from the standard. However, after reading all the messages I wanted, I did SHIFT-ESC to mark ALL messages as read and the Threads was dimmed. FWIW. YMMV.

I have the app, and no premium. So maybe the message limit is the key

Yeah. That seems to be the issue.

@U01E6385CN8 I am not clear what you mean by "and no premium". As far as I know, Slack does not offer the free-service workspace like Clojurians where some users can pay for it, but others get it for free. Last I checked (2020) they offered either "everyone gets free with 10,000 limit on searchable message history" or "some one legal entity pays for everyone to get unlimited search history", and Clojurians Slack is the first one.

@U0CMVHBL2 A lot of people find Slack's messaging very confusing and assume it means they, personally, don't have a "premium" account -- rather than the two choices you just outlined.