I spent some time figuring out some useful search queries for Gmail that can also be used as filters (e.g. to assign label):

• Issues: subject:(Issue -PR)
  • might miss some issues that include "PR" in the subject
• PRs: subject:(PR)
  • might include some issues that include "PR" in the subject
  • subject:(PR -Issue) could be used instead
    • it will miss PRs that include "issue" (somewhat more common)
• Merged PRs: subject:PR ((Merged AROUND 1 "into main") OR (Merged AROUND 1 "into master"))
  • You can add entries for other branches too (e.g. (Merged AROUND 1 "into 3.10"))
  • Still has a few false positives
    • "I merged into main" or "I merged this into main" match (because "into main" is within 1 word of "merged")
    • "I merged this PR into main" doesn't match (because "into main" is 2+ words away from "merged")
• Closed issues: subject:Issue "closed"
  • quite unreliable -- it matches any issue that contains the word "closed"
• Closed issues (by a linked PR): subject:Issue "closed AROUND 1 via"
  • pretty reliable, but only works for issues closed automatically by a linked PR

Note:

• All queries should include from:(notifications@github.com)
• subject:Issue and subject:PR don't work for old Issues/PRs
• See also this gist with some additional suggestion

This should be documented somewhere in the devguide, so that other people could easily copy them to set up filters.