They can ask for this document anytime during the process even after getting Sponsorship approval or just before PA getting PPR. At that time they will be checking the 3 years preceding the time of the request made for this document which can be 2020, 2019, (inclusive 2018, 2017, 2016) considering the request made today.
https://www.canada.ca/en/immigratio...mily-class-assessing-sponsor.html#finReassess
"You are required to maintain income that meets the MNI value for your family size on the day you sign and submit the application until the application has been processed and your family member has become a Canadian permanent resident. It is important to ensure that you continue to meet this requirement as financial ability to support your relative can be reassessed at any time during the application process. If your income falls below the MNI, your application could be refused." Source:
https://immigrating-to-canada.com/sponsor/family-member-sponsorship/super-visa-sponsorships/