as a general rule, is it required (or even advisable) to apply all available patches for the current release before starting the upgrade process?
For example, before upgrading from 8.9 to 9.1, should I first apply the latest 8.9 application maintenance packs and product patches, or is that unnecessary. Or even more specific, at PeopleSoft Financials 8.9 MP8/Tools 8.49.xx is it ok to go straight for "required for upgrade" patches? Oh and tools upgrade to fulfill requirements ofc.
As long as you match the certified configurations on your source as well as on your target system, pay attention on the chapter "Updates and Fixes Required at Upgrade" in the note you mentioned. The pdf explain what you need and what you don't need.
As for the Peopletools patch level required, if nothing is there, then your certified source system should be ok. Read also "Certified Upgrade Path" where you can find source and target application/peopletools path upgrade.