As quoted and linked a year ago by
@spyfy, in the post referenced and linked above, the relevant IRCC PDI states:
"Also, applicants who submit their application early in the calendar year would not have been required to file taxes yet for the previous calendar year."
Thus, those who apply between January 1 and sometime in April, can check "No" not required to file for the preceding tax year. Thus, for now and at least into April, a "No/No" response (not required to file and did not file) for 2019 is OK and should count as one of the three required years of compliance with CRA tax filing obligations.
What is technically OK is not necessarily the prudent approach. Thus, even though technically "No/No" is OK for 2019 until April 30, 2020 (and perhaps until June 15th for those of us self-employed), probably best to have three other years that show compliance with the filing obligations. And the closer it gets to the tax-filing deadline (which triggers required-to-file status for the previous tax year) the more prudent it will be to file before applying for citizenship and thus be able to check "Yes/Yes" for 2019. (That is, for January and February, "No/No" should be a perfectly OK response for the previous tax year; by March the prudent PR might want to wait until after filing last year's taxes to make the citizenship application, for the sake of appearance, recognizing it is not necessary.)
Similarly, technically an applicant meets the requirements so long as there are no more than two years for which the applicant checks "Yes/No" (all other responses reflect compliance with the tax filing obligation for a given year) . . . but the best impression will be made if there are NO years for which the applicant checks "Yes/No" . . . that is, obviously, it makes a better impression if there have been NO years in which the applicant failed to comply with tax filing obligations, even though that is NOT what is required to be eligible for citizenship. (In general, many appear to put rather too much weight on meeting the technicalities and overlook the influence equitable factors,
who-deserves-what factors, can have; appearances can matter.)