Oh nice, I have just the tables for that! Billions of well-compacted rows with a big ol' timestamp on them. Btree indices are almost as big as the table itself...
Note that if you delete entries in non sequential order (aka not at the start-only or the end-only) and continue inserting, BRIN index becomes much less effective. Its very much predicated upon the concept of append-only, as postgres filling deleted rows with new rows throws off the physical layout part of how BRIN works
I don't delete anything in those tables, however I do one update on 9/10 of the rows, at least.
I seem to remember there was a way to reorder rows in a table, maybe that could be useful.
You can check if your data remain correlated after your updates. It's possible they do: SELECT correlation FROM pg_stats WHERE tablename = 'mytable' AND attname = 'mytimestamp';