New features / improvements:
- Fix #759 #1109 #1110 by adding custom properties in ReceiverEvent and Context objects (#1177) - Thanks @seratch!
- Fix #860 Enable developers to customize the built-in receivers more (#1183) - Thanks @seratch!
- Fix #1181 Add port property to installerOptions in the HTTPReceiver (#1184) - Thanks @seratch @M1kep!
- Add port property to installerOptions in the HTTPReceiver (#1181) - Thanks @srajiang!
- Add context to global error handler (#525) - Thanks @raycharius!
- Fix #1098 next() is optional in middleware in TypeScript (#1099) - Thanks @seratch!
- Fix #1148 - Adjust the app.message listener interface in TypeScript to compile the examples in documents (#1185) - Thanks @M1kep!
- BlockAction interface does not include state despite state being present in actual object (#1141 #1144) - Thanks @seratch @Richard-PTT!
- Add more information to unhandled incoming request logging (#1143) - Thanks @misscoded!
- Bump axios version to 0.21.2 or higher for better security (#1162) - Thanks @xmariopereira!
- Fix the v3.8.0 publish error (#1194) - Thanks @brianjychan
Document updates:
- Japanese document updates (#1047 #1152 #1131 #1154 #1165 #1163 #1166 #1169 #1175) - Thanks @wongjas!
- Fix the logging example so that it is valid JS (#1172 #1174) - Thanks @filmaj!
- Fix documentation about state verification option (#1168) - Thanks @stophecom!
- Fix call in AWS handler to match example (and be correct) (#1190) - Thanks @sirctseb!
- using directMention() documentation doesn't seem to be correct in docs (#1148) - Thanks @O-Mutt!
- Clarify /slack/events path requirement (#1153) - Thanks @mars!
- Update Japanese docs to apply token rotation (#1009) changes (#1014) - Thanks @misscoded!
- Update slugs in document pages (#1161) - Thanks @srajiang!
- Japanese document updates (#1067 #1137) - Thanks @seratch!
- Minor updates related to #1046 (#1047) - Thanks @seratch!

Here is the list of all the issues / pull requests included in the release: https://github.com/slackapi/bolt-js/milestone/15?closed=1