Removing yourself from a list has been possible for a long time (since at least 2013) by blocking the list owner, then unblocking, but was only documented late 2019 in [WayBack] How to use Twitter Lists:
A Twitter List is a curated group of Twitter accounts. Create your own or subscribe to a List created by someone else to view a streamlined timeline.
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How to remove yourself from a List
You can view which Lists you are a member of through your Lists tab. To remove yourself from a List you will need to block the creator of that List.
It was public knowledge though, for instance documented at [WayBack] How journalists can remove themselves from Twitter lists – & why it matters – Poynter, which also documents this:
How can I find out which Twitter lists I’m on?
When using Twitter on the web, click on lists. You will arrive at “Subscribed To.” Next to that heading, you’ll see “Member Of.” Click on it to see the Twitter lists that include you as a member. This list of lists is chronological starting at the bottom — the first list you see at the top will be the one that most recently added you.
For me this is twitter.com/jpluimers/lists/memberships
The same trick also works when you want to have someone un-follow you:
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