Technically all applicants can still check "NO" not required to file for the tax year 2018 . . . since no one is required to file before April 30. (In other topics
@spyfy has more fully addressed early in the year applications and the previous tax year, including reference to what IRCC states about it.)
If the applicant has already filed a 2018 return (noting that the earliest anyone could file a 2018 return was just a couple days ago), OK to check "yes -- yes" and it is not as if IRCC will have a problem if the applicant checks "yes -- no" (required but not filed) for 2018, as long as there are at least three other tax years for which the applicant indicates compliance with the obligations to file a return.
OBSERVATION ABOUT LISTING FEWER THAN FIVE TAX YEARS -- APPLICANTS WHO ARRIVED IN CANADA LESS THAN FIVE YEARS AGO:
Technically a failure to list the full FIVE previous tax years in the application is a mistake. As others have stated,
list all FIVE tax years prior to the year in which the application is made. That includes listing tax years prior to any Canadian ties if the tax year is within the previous five.
For tax years prior to coming to Canada, entering "no -- no" not only is the proper way to complete the form, but then these years count toward the necessary three years of compliance. (Assuming that the individual had no other circumstances requiring the filing of a Canadian tax return those years, such as NO Canadian source income.)
IRCC is NOT in the GOTCHA game. A technical mistake of this sort (leaving out years prior to arrival in Canada) may very well be overlooked. Almost all applicants make some mistake in their application. IRCC is more interested in verifying whether the applicant meets the qualifications than it is in technicalities (that said, certain technicalities impose mandatory requirements, such as technically falling short of 1095 days presence precludes a grant of citizenship and cannot be waived or disregarded by IRCC).
CAUTION: Remember that AOR ONLY means the application is complete. Anecdotal reports reporting that an applicant did this or that, and still got AOR, so that suggests it was OK to do that thing,
SHOULD BE IGNORED. Assessment of actual qualifications comes much later in the process, well after AOR, when a processing agent in the local office begins preparing the file for the PI Interview and test. Again, all AOR means is that the applicant's application is complete and will be processed.