I think it *should* be enabled for planning, since that makes the default easier to understand and document, and it makes a user's use of "explain" easier.
I’d be keen to see BUFFERS off by default with EXPLAIN, and on by default with EXPLAIN ANALYZE.
The SUMMARY flag was implemented that way, which I think has been easy enough for folks to understand and document. In fact, I think the only BUFFERS information goes in the “summary” section for EXPLAIN (BUFFERS), so maybe it makes perfect sense? If it would be great if that made implementation easier, too.
In any case, thank you all, I’m so glad that this is being discussed again. It’d be so good to start seeing buffers in more plans.