Cheers @straw_collector, setting up a new profile did work for me!
I went to "Settings" / "General" / "Manage profiles" and clicked "New Profile".
I copied nothing from the "Default" profile, so started out with no extensions etc.
I went to https://old.reddit.com and saw the promoted stories, then added a minimum custom rule:
reddit.com##.promotedlink
After that the promoted posts didn't show.
I'll add back in my usual extensions and see if one of them breaks things again.
Notes about my non-working default profile
The extensions I had installed were the following, though I originally had uBlock Origin installed but had uninstalled it a few days ago.
- Bitwarden
- Nitter Redirect
- Old Reddit Redirect
- Sponsor Block
- Surfingkeys
- Unhook for Youtube
My custom filter rules were being persisted to disk - I could add/edit a custom filter, and see the changes in ~/Library/Application\ Support/Orion/Defaults/ContentRules/ContentRuleList-userFilters - it's a binary file, but there is a JSON fragment at the top, which holds the custom rules.
I got the idea to check the userFilters file from a similar bug report at https://orionfeedback.org/d/6586-custom-filter-rules-are-sometimes-not-transposed-into-contentrulelist-entries